Even in advanced economies where family sizes has declined over the last 50 years, most children and youth today have at least one sibling. Despite this ubiquity and the known mental health benefits from a supportive sibling relationship, research on emerging adulthood has rarely examined if siblings may contribute in reducing psychological distress. Hence, the overarching question of my thesis was to examine mechanisms (protective, compensatory, and promotive) through which sibling support may mitigate the risky effects of parent and peer alienation on stress and psychological distress in emerging adulthood. The literature in childhood and adolescence appears to suggest that siblings are protective and foster mental health. However, the beneficial effects of siblings have rarely been theorized at the beginning of adulthood, notably in college students who are known to develop more independence from their family. Nevertheless, I was able to ground my thesis in two well-established theoretical frameworks. First, the stress-buffering hypothesis, which postulates that perceived social support should counteract the adverse effects of stress on mental health. Second, concepts of developmental psychopathology, which operationalize the notions of risk, protective, promotive and compensatory factors. Four empirical studies, presented across three articles, were conducted to address the overarching question of this thesis. In Article 1, which presents the first study, we examined the protective role of a supportive sibling climate (i.e. supportive experiences across all siblings) in the hypothesized moderated-mediation model across three independent samples of emerging adults (N=310, N=259, and N=416) using latent moderated structural equation modeling (LMSEM). The hypothesized moderated-mediation model examined the protective (moderating) role of sibling support in a mediation model of the effect that parent and peer alienation has on psychological distress through stress. Inconsistent with our hypothesized model, sibling climate did not moderate the paths linking parent and peer alienation to stress and psychological distress. Nonetheless, in support of the risk mediation model, general stress partially mediated the link between parent (and not peer) alienation and psychological distress. This first study underscored that while not protective, a supportive sibling climate may be a promotive of mental health, and that parents may have an enduring influence during emerging adulthood given that experiences of alienation in these relationships was indicative of more stress and psychological distress. In Article 2, which reports the second and third studies, we used the same moderated-mediation model to examine whether a supportive sibling relationships with one's closest sibling could have protective effects among emerging adults (N=789 and N=325). Additionally, the mediating role of two stress-related mechanisms we also tested: (a) stress in general (Study 2) and interpersonal stress specific to parents and peers (Study 2 and 3). Contrary to our hypothesized protective effects, these studies found mixed effects in the form of both accentuating and attenuating influences of sibling support in the links among alienation, stress, and psychological distress. In Study 2, a worsening effect of siblings suggested peer alienation was related to more stress but only when emerging adults receive more support from a sibling. At the same time, stress from peer alienation was related to less psychological distress in the context of more sibling support. In study 3, a buffering effect of siblings indicated parent alienation was related to less psychological distress in the context of higher levels of support from siblings. Partly consistent with the proposed mediation model, both studies found parent and peer alienation were associated with more psychological distress and this path is mediated by stress in general and not interpersonal stress (only Study 2). Overall, these studies imply sibling support only partially and rarely buffers the link between experiences of alienation and psychological distress as these protective effects failed to replicate across the studies. Given the limited evidence for the protective role of siblings in the moderated-mediation model, in the last article, which reports the fourth study, we focused on the compensatory role of siblings on the development of stress and psychological distress over a semester, specifically in the context of parent and peer alienation among emerging adults (N =234). The 3-month longitudinal findings suggest psychological distress and stress decrease over the course of three academic months. Furthermore, parent and peer alienation nor sibling support were not predictive of stress or psychological distress over time. Taken together, we found supportive sibling relationships, whether across multiple siblings or with one sibling, may not have a protective or compensatory effect against stress and psychological distress when accounting for experiences of alienation from parents and peers in emerging adulthood. Thus, the benefits of siblings in emerging adulthood might at best be promotive in the context of alienation. Furthermore, parent and peer alienation were not identified as risk factors in the longitudinal study, while they were consistently associated with stress and psychological distress in the cross-sectional studies. Finally, unexpectedly, stress and psychological distress were found to decrease over a semester suggesting emerging adults might be more resilient to manage the challenges of a semester than often claimed. In conclusion, to answer the overarching question of this thesis, these findings suggest emerging adults might not be able to rely on their sibling relationships to protect them from psychological distress when faced with stressful experiences of parent and peer alienation. Rather, their supportive siblings might only mitigate psychological distress in the absence of any of these harmful experiences.