Date of Award


Degree Type


Degree Name

Doctor of Philosophy


School of Psychology and Social Science


Health, Engineering and Science

First Advisor

Dr Greg Dear

Second Advisor

Dr Susan Carruthers


The sibling relationship is unique in that it is relatively egalitarian, ascribed, and can be the longest-lasting across the lifespan. Siblings can act as supports for one another during major life events, both in childhood and adulthood. Siblings can also be a source of significant stress. The literature on family coping indicates that there are significant impacts to family members’ well-being from dealing with stress and strain that result from a family member’s drug use problem. However, researchers have not investigated the impacts on adult siblings despite the importance and uniqueness of sibling relationships. The broad aim of this research was to develop a theory of the adult sibling relationship when one sibling’s drug use impacts significantly on the quality of the relationship. Phenomenological interviews with 25 adults with a sibling with an illicit drug use problem were analysed using grounded theory. A provisional model and theory was developed from the first phase of data analysis which was then consolidated in the second phase. Two case studies were drawn from the pool of participants to illustrate how the model and theory developed here could be applied to assist a forensic evaluator in child protection and family court matters. Several themes related to stress and distress, coping, and support were identified. Adults were more likely to use social support rather than access professional services. Adults were found to experience distress comparable to parents or partners. However, they were likely to feel distress both from direct impacts from their sibling and from witnessing the impact on parents and other family members. Adults who characterised their sibling relationship as warm and close since childhood experienced a cycle of engagement and support of the user followed by detachment and bounded relationships. These adults were particularly influenced to engage in support due to a high sense of obligation to care for their siblings and also experienced difficulty disengaging from their sibling. Adults whose sibling relationships were characterised by high conflict (rivalry) or indifference since childhood felt less obliged to engage in support for their sibling and maintained clear boundaries. Adult siblings reported a belief that siblings have more freedom to detach from a user sibling than parents have freedom to detach from a user child, especially if more vulnerable family members required protection from the user. Forensic evaluators can use the theory as a guiding framework when a sibling is a litigant or witness in child-protection or Family Court matters. The findings also inform clinical practice in terms of the psychological needs of this population, such as issues with adjustment, grief and loss, stress, general coping, and the impact of protracted and disruptive life experiences as a result of having a sibling with an illicit drug use problem.