Science mentoring programs are powerful opportunities for youth to develop conceptual knowledge, undertake authentic practices, and have impacts on their science-related identity work. Here, we use design-based research to understand how a university-community partnership expanded upon traditional mentoring structures to facilitate "relational equity" (DiGiacomo & Gutiérrez, 2016) through distributing expertise and building relationships between participants. We analyzed qualitative data from 2 years of the STEM OUT mentoring program to develop claims about the elements of program design that led to distributed expertise and building relationships. Key findings include the need to design structures that position all participants as having expertise, highlight relationship-building as integral for youth-scientist interactions, and facilitate equitable power dynamics. Our findings are articulated as design principles for other youth-scientist mentoring programs, with the goal of broadening participation in the sciences by redefining not only who participates, but also what counts as science.