Background Recruitment to clinical research studies can prove complex. This is particularly true of mental health research, given factors such as confidentiality, capacity and consent, or when attempting to recruit family members as opposed to service users themselves. Aim This study investigated the challenges experienced and strategies employed in the recruitment of siblings of people with first episode psychosis using Early Intervention in Psychosis Services (EIPS) in England. Methods As part of a randomized controlled trial (RCT) of an e‐health intervention for siblings, we conducted a process evaluation study whereby semistructured interview was undertaken with clinical and research staff involved in recruitment of siblings. Data were analysed thematically. Results Twelve participants from six EIPS were interviewed. Data analysis revealed seven key themes: (i) limited comprehensive family data available; (ii) data governance and consent issues; (iii) organizational factors; (iv) convoluted recruitment methods; (v) concerns about service users' opinions; (vi) fluidity in siblings' needs and expectations; and (vii) strategies to enhance recruitment. Conclusions Recruitment challenges identified in this study concerned administrative, organizational, process and attitudinal issues. These are similar to other studies recruiting mental health service users as well as family members. Failure to recruit to target implies that studies are underpowered to detect potential statistically or clinically meaningful changes. Future studies should establish how best to enhance family inclusiveness in clinical practice and research.