We must forgive the sister who stroked her brother's genitals.

It sounds like you're referencing a situation that might involve complex emotions and difficult boundaries. Forgiveness can be a personal journey, and it often relates to understanding the context, motivations, and consequences of actions. If this situation involves harm or violation of trust, it may be important to seek resolution that prioritizes emotional safety and healing for everyone involved. If you feel comfortable, sharing more context could help provide better guidance or support for navigating such a situation.