Eating disorders (EDs) are serious mental health problems that cause disturbances related to food intake and lead to major concerns about weight and body shape. In addition, they have an impact on social functioning and lead to psychological symptoms such as depression and anxiety, social isolation, interpersonal difficulties, low self-esteem, and reduced autonomy. These disorders share a high comorbidity with personality disorders (PD) that ranges from 27-93%, leading to increased severity and worse quality of life. Families of people with ED perceive a high burden of the illness and report mental health problems. Existing intervention programs specifically targeting caregivers of people with ED are based on psychoeducation, systemic cognitive-behavioral therapy, although specific treatments for family members of people with ED and PD are still scarce. However, a skills training program called "Family Connections" (FC) was developed by Hoffman and Fruzzetti's group for borderline personality disorder (BPD). It is a program consisting of 12 two-hour sessions that take place weekly. It is divided into six modules that include psychoeducation about the disorder and how it affects family functioning, and skills adapted from Dialectical Behavior Therapy, such as mindfulness, emotional regulation, validation, radical acceptance, and problem management, among others. In addition, all the modules include specific practical exercises, video examples, and homework. There is also a forum where family members build an emotional support network. So far, the results of this program in non-randomized clinical trials show significant decreases in the subjective experience of burden of illness, perceived distress, and depression, as well as improvements in coping strategies. The aims of this study are the following: (a) to adapt and test, in the Spanish population, the modules of the FC intervention protocol, designed to be applied specifically to relatives of patients with ED and PD; (b) to test the efficacy of FC in reducing the perceived burden of illness and clinical symptomatology in relatives of patients with ED and PD and improving family relationships and quality of life; (c) to test its efficiency, understood as the acceptance of the intervention program by the participants and clinicians, and demonstrate its viability; (d) to carry out an effective dissemination of this protocol. This paper presents the study protocol. The study design consists of a two-arm randomized controlled trial with two conditions: Family Connections (FC-ED \& PD) or Treatment as usual optimized (TAU-O). Participants will be family members of patients with eating disorders and personality disorders (or dysfunctional personality traits) according to the 5th version of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5). The caregivers' primary outcome measures will be the BAS and a Family-Patient Critical Incident Record. Secondary outcomes will be the EDSIS, FAD-GD, MEMs, MQLI, DASS-21, DERS, and QoL. The patients' primary outcome measures will be a Family-Patient Critical Incident Record, the EAT-26, PHQ-9, OASIS, VIRS, and LEAP. Participants will be assessed at pre-treatment, post-treatment, and twelve months follow-up. The intention-to-treat principle will be used when analyzing data, using mixed-effects models with full information and maximum likelihood estimation.