A difficult situation with my younger brother and my cousins

Salam everyone. I'm writing this with a heavy heart. I'm a 20-year-old sister, and I'm really lost about what to do. My younger brother is staying with relatives for the holidays, along with my little sister. They have three daughters, two of them are very young. One of my cousins told me she saw him doing something inappropriate while one of her younger sisters was sleeping. It seems her brother might be involved too. All of them are minors, and the one who told me is just a child herself. I'm worried because he has done similar things to me and another cousin in the past when we were asleep. I thought he had stopped-we talked about it two years ago-but it seems it's happening again. We can't tell our parents because both our fathers work far away, my mother has anxiety and depression, and my aunt lives in a joint family so she can't easily act. I feel stuck. I'm far from them and can only talk to him, but he'll likely avoid me once he knows I'm aware. I don't want to involve the police-it would break my parents, and honestly, I'm scared. But I can't bear the thought of my cousins and sister going through what we did. What should I do? Edit: My father recently had surgery, and my uncle has diabetes with very high sugar levels. Both doctors said they must avoid stress. My family is traditional and often sides with boys over girls. I've asked my cousin to speak to our uncle-he's the only one who might believe us. Our grandmother tends to blame the girls, so that's not an option. Please make dua for us.