The program condition includes 12 weekly 90-minute afterschool group sessions for siblings. Sessions are structured as psycho-educational groups and include social interactional activities, role-playing, discussion, and didactic presentation. The focus is on sibling relationship skills, cognitions, and activities. During a total of 3 family nights, parents attend with their children. Part of the family night session involves parents and children together. Another part of the session involves parents being separated from children. Family Nights promote parents' understanding of sibling relationships, review concepts, provide strategies for parental support of siblings, and teach parents skills for dealing with sibling problems. Family Nights include dinner and last 2 hours.

The Contact-Equivalent Attention Control condition includes 12 weekly 90-minute afterschool group sessions for siblings led by two co-leaders. Students work on educational games and activities. Groups begin with an icebreaker and continue with games and projects. This condition also includes 3 family nights, where parents attend with their children. Activities of the Family Nights include children showing their parents the activities they have been engaging in during the sessions. Family Nights include dinner and last 2 hours. Part of the family night session involves parents and children together. Another part of the session involves parents being separated from children; during this part, parents will break out with one group leader, and siblings will work with the other group leader.

Universal prevention program promoting positive sibling and family relationships conducted in an afterschool setting

The measure assesses parents' perceptions of three constructs: Parents' positive guidance, authoritarian control, and involvement (McHale et al., 2000; Updegraff et al., 2016). For each subscale, scores range from 1 to 5. Higher subscale scores indicate more positive guidance (i.e., better outcome), higher involvement (i.e., better outcome), and higher authoritarian control (i.e., worse outcome). Subscales are not combined into a total score.

The measure assesses parents' perceptions of one construct: Parents' ratings of children's supportive and prosocial behaviors (Stormshak, Bellanti, & Bierman, 1996; Updegraff et al., 2016).

Scores range from 1 to 6. Higher scores indicate parents' perceptions of target child's more prosocial behaviors toward their sibling (i.e., better outcome).

The measure assesses siblings' ratings of intimacy/emotional support (Blyth & Foster-Clark, 1987). Scores range from 1 to 5. Higher scores indicate target child's perceptions of higher sibling intimacy and closeness (i.e., better outcome).

The measure assesses parents' ratings of sibling conflict and negativity (Stocker & McHale, 1992). Scores range from 1 to 5. Higher scores indicate parents' perceptions of target child's greater negativity toward their sibling (i.e., worse outcome).

The measure assesses children's ratings of sibling conflict and negativity (Furman & Buhrmester, 1985). Scores range from 1 to 5. Higher scores indicate target child's perceptions of higher sibling conflict and negativity (i.e., worse outcome).

This measure assesses children's ratings of two constructs: social self-efficacy and emotional self-efficacy (Muris, 2001). For each subscale, scores range from 1 to 5. Higher subscale scores indicate greater social self-efficacy (i.e., better outcome) and greater emotional self-efficacy (i.e., better outcome). Subscales are not combined into a total score.