Mental disorders are at the root of many clinical and behavioural issues. The definition of severe mental disorders (SMD) includes factors of a psychosocial, temporal, and disabling nature, as well as cognitive, affective, and social alterations that have a major impact on the lives of patients and their families (Roldán et al., 2015). These problems affect the everyday lives and well-being of everyone involved (Polo-López et al., 2016). Such disorders include the schizophrenia spectrum and other psychotic, depressive, bipolar, and related disorders, including personality disorders.

In Spain, these diagnoses are the fourth most common reason for need of care from relatives and non-professionals (Navarro et al., 2019). SMD do not affect every family member in the same way. Parents and caregivers are the most affected by issues relating to quality of life, coping skills, distress/anxiety, and burnout (Caqueo-Urízar et al., 2012; González-Fraile et al., 2018; Lahera et al., 2020; Polo-López et al., 2016; Roldán et al., 2015). Meanwhile, siblings report feeling mixed emotions, including love, pain, anger, envy, guilt, and shame, stemming from challenges associated with grieving the loss of a “healthy brother or sister”, their fear of also inheriting an SMD (Stålberg et al., 2004), psychological distress (Plessis et al., 2020), stress (Seeman, 2013), the use of coping strategies (Avcıoğlu et al., 2019; Friedrich et al., 2008), subjective burden (Avcıoğlu et al., 2019; Pierazzuoli et al., 2020; Shivers et al., 2023), uncertainty about their future as primary caregivers as their parents age (Smith et al., 2007), and the need for support for their own psychological needs (Amaresha et al., 2014). However, to our knowledge, there are no empirically validated treatment programmes tailored to this cohort and their needs that mitigate the effects of their experience of their sibling’s SMD outside of the healthcare system (Friedrich et al., 2008; Nechmad, 2000).

Family intervention programmes in the field of SMD (Lahera et al., 2020; Navarro et al., 2019) have so far yielded significant benefits for both caregivers and patients. These programmes have been shown to improve treatment, care, and recovery (Sampietro et al., 2023), outperforming cases without family interventions (Vallina Fernández & Lemos Giráldez, 2000).

However, a review of the participants in these psychological interventions reveals a high level of participation by parents, particularly mothers, who are usually the primary caregivers and typically shoulder the greatest burden. Siblings tend to benefit only from psychoeducation, which is mainly delivered by nursing professionals (Sin et al., 2015). As a result, the lack of interventions targeting siblings of people with SMD within the mental healthcare system (at least in terms of empirical evidence) means that siblings are often the unsung (Friedrich et al., 2008).

Third-wave therapies (also known as contextual-behavioural therapies) represent an innovative approach to psychotherapy that goes beyond traditional behavioural and cognitive-behavioural models. These therapies focus on promoting psychological flexibility and values-based living, with an emphasis on the context in which thoughts, feelings, and behaviours are experienced. This study presents an intervention protocol based on third wave of cognitive-behavioural therapy (CBT) and consisting of three interrelated blocks: (1) recognition, awareness, and openness, (2) addressing psychological content, and (3) behavioural strategies. This clinical protocol emphasised issues such as mindfulness, acceptance, emotions, values, goals, cognitive defusion, psychological flexibility and the therapeutic relationship, following Hayes and Hofmann (2017).

Tables 2 and 3 show the means, standard deviations and effect sizes (η²_p and Cohen’s d), as well as the between-group differences, assessment time and Time x Group (T x G) interactions of the various psychological variables assessed.

The aim of this study was to examine the effectiveness of a group-based psychological intervention utilizing techniques from the third wave of cognitive behaviour therapy on the self-esteem, perceived stress, perceived health problems, coping strategies, psychological inflexibility, and caregiving experience in siblings of patients with SMD. The findings suggest that the programme led to increases in self-esteem, the use of problem-focused adaptive coping strategies and positive perceptions of health problems. There were also decreases in perceived stress, the use of problem- and emotion-focused maladaptive coping strategies, negative appraisals of the caregiving experience, and psychological inflexibility. By contrast, psychological difficulties (e.g., psychological inflexibility) persisted and even deteriorated in those who did not participate in the programme (CG). We can therefore conclude that the data support the main hypothesis of the study.

One of the more significant changes concerns “psychological inflexibility”. These changes reflect the programme’s content and exercises, which are designed to help participants better understand their own psychological processes and learn new ways of dealing with distress and psychological challenges. The programme teaches participants to adopt a dispassionate perspective when observing experiences (e.g., having a sibling with an SMD), focus on the present, accept negative emotions, distance themselves from thoughts that exacerbate distress, and behave in accordance with their personal values. Interventions of this kind have yielded positive outcomes in families of people with SMD (e.g., Roldán et al., 2015) and in patients with some form of mental disorder (e.g., Salazar et al., 2022).

The “self-esteem” variable also experienced a significant increase following the intervention. This is another notable finding, as self-esteem tends to be affected in family members of people with an SMD (Navarro et al., 2019), highlighting the importance of reinforcing it through psychiatric rehabilitation programmes. Although the intervention programme did not specifically target self-esteem, the change in the way individuals judged themselves (and their self-worth) could be attributed to acceptance, the development of a value system and support for actions guided by such values. These findings cannot be compared with other studies of siblings of people with an SMD, as no such studies exist. However, other research on group-based third-wave of CBT has documented its benefits for the self-esteem of people with other psychological problems (e.g., Salazar et al., 2022).

Participants in the study also reported improvements in terms of “perceived health”. There were significant reductions in somatic symptoms, anxiety, and depression. These types of health problems and vulnerability to them have been reported in siblings of people with SMD (Friedrich et al., 2008). Given the prevalence of depressive and anxiety disorders in Spain and worldwide, tackling these issues should be a priority in mental health care. It is important to reiterate that there are no previous studies of interventions involving a sample such as ours. However, studies have been conducted with parents/caregivers of people with SMD whose symptoms of anxiety and depression have been alleviated by psychological interventions like this one (Navarro et al., 2019; Roldán et al., 2015).

The effectiveness of the psychological intervention programme is demonstrated by the changes observed in participants’ “perceived stress” levels. The results show that siblings can learn to cope with stress, especially in complex situations such as those brought on by SMD. It is widely recognised that stress is the product of a person’s interaction with their environment. In this context, the programme is designed to build individuals’ capacity to better understand how psychological responses are triggered and then learn to control them. It is evident that interventions are needed to help siblings of people with SMD to manage and reduce stress (Friedrich et al., 2008; Nechmad, 2000; Roldán et al., 2015), and we now have a tool that fulfils this requirement.

Regarding “coping strategies”, the changes observed after treatment indicate an improvement in the use of adaptive coping strategies, especially those focused directly on the problem, alongside a significant reduction in the use of maladaptive strategies. The intervention programme directly promotes adaptive coping by incorporating some of the techniques that help with emotional regulation (e.g., mindfulness, breathing, and defusing dysfunctional thoughts). It also assists in overcoming problems and challenging situations. Previous research has highlighted the importance of problem-focused coping for people’s well-being (Avcıoğlu et al., 2019) and the role of emotion-focused coping in caregiving (Avcıoğlu et al. 2019; Friedrich et al., 2008).

One last positive finding relates to the change in siblings’ appraisals of their “caregiving experience” with patients with an SMD. The results suggest that their involvement in the programme led to an increase in positive experience and a decrease in negative experience. A particular highlight of the programme is its emphasis on exercises and the use of metaphors to help participants express their emotions, reflect on how to behave in a way that is consistent with their values and fully enjoy the here-and-now without judgement. The success of this approach is clearly reflected in the improvement in the psychological flexibility of the participants. In our opinion, this helps them to be less intolerant of the situations they face due to the severity of the disorder and thus improves their appraisal of the experience.

This study has yielded several positive outcomes and has pioneered research into how siblings of people with SMD can benefit from psychological interventions specifically tailored to their needs (e.g., self-esteem, perceived health, and perceived stress) and to issues that indirectly affect the patient. These include changes in their caregiving experience, as well as coping and problem-solving strategies. To date, most interventions involving relatives of patients with SMD have targeted parents (Amaresha et al., 2014). Although the disorder affects the whole household, siblings have been largely overlooked by healthcare services. Therefore, bearing in mind the empirical evidence on the impact of parental interventions on the recovery/rehabilitation of patients with an SMD (González-Fraile et al., 2018; Navarro et al., 2019; Roldán et al., 2015), we contend that the treatment programme examined here will benefit not only the siblings, but also the patients themselves. This may provide families with strategies for mental health prevention, detection, and support.

There are some limitations to this study. One such limitation is the way in which the intervention and control groups were set up. Although all siblings agreed to take part in the study, not all could be included in the IG due to time constraints or scheduling problems. These participants were assigned to the CG, potentially introducing bias into the research findings. Another limitation was the lack of any kind of follow-up to determine whether the benefits of the programme were sustained in the medium and long term. Finally, we believe that there is a need to enrol more participants in the intervention programme to achieve greater generalisability.

In conclusion, the group-based psychological intervention using third-wave of CBT in siblings of patients with SMD was found to be effective. This is evidenced by improvements in adaptive coping, self-esteem, and positive caregiving experience and reductions in maladaptive coping, perceived health problems, stress, and psychological inflexibility. This optimises the time and care that professionals can devote to healthcare service users.