Although parents of siblings play a crucial role in the development of both the positive and negative aspects of their children’s relationships, relatively few parenting programs specifically target sibling conflict alongside sibling warmth. To bridge this gap, the current study reports the outcomes of a randomized controlled trial evaluating a brief parenting intervention focusing on improving sibling relationships. A total of 74 parents reporting concerns about sibling conflict were randomized to either the Triple P brief discussion group intervention condition (n = 37) or the waitlist control condition (n = 37). Parents were predominately Caucasian, female, from a middle-income background. Parents completed a range of self-report measures examining the quality of the sibling relationship, parenting practices, family functioning, and child emotional and behavioral problems. Across treatment groups, participants showed improvements on almost all measures of sibling conflict, sibling warmth, child emotional and behavioral problems, parenting practices, and parenting confidence. Participants in the treatment group also experienced a greater reduction in the amount of help they wanted to manage sibling warmth, relative to participants in the control group. These results suggest that a low-intensity parenting intervention may only have a small effect on improving sibling relationships. Additional research is needed to determine how to broaden program efficacy for sibling conflict and parental adjustment, for families from diverse levels of income and family composition. Trial registration: ANZCTR 365567.