Software ecosystems (SECO) comprise multiple actors collaborating around a common technological platform, typically governed by a keystone organization. Power relationships define how decisions are made, who holds influence, and how value is shared. When power becomes concentrated, it can undermine social sustainability goals such as fairness, inclusion, and trust. Despite its importance, this topic remains underexplored in software engineering, particularly in applied research. This study investigates how power relationships affect social sustainability in SECO and proposes PReSS-SECO (Power Relationship Effects on Social Sustainability in Software Ecosystems), a framework to diagnose and mitigate power imbalances. This framework includes a conceptual model, a diagnostic tool, and a feedback mechanism for governance improvement. These artifacts will be developed and evaluated through Design Science Research (DSR), based on literature reviews, a participatory case study, focus groups with experts, and validation por evaluation. The research aims to offer information technology (IT) managers a practical means to assess and address power asymmetries, fostering more inclusive and sustainable SECO governance.