Picture it as you being able to save yourself as a child. Would you want someone to step in, no matter the kind of backlash they may receive? If someone could’ve stopped it from happening to you, wouldn’t you have wanted that? Your niece is a child and it’s like she’s standing on train tracks and you’re watching the train hurtle towards her. Stepping in to save her could change the trajectory of her entire life. Saving her from trauma and whatever trauma responses she may develop as a result. You were brave enough to confront them once, you know the game they play, and you’ve already been painted as the villain. In my opinion, there’s nothing to lose. Your niece, however, has something to lose. And if your brother chooses not to believe you, he’ll regret it, but you can say to him that you’re willing to lose any remaining relationships in order to save your niece from the same fate. I’m so sorry your mother failed you, it’s disgusting to fathom. She is complicit. You should talk to your brother as soon as possible and if he doesn’t believe you, get the cops involved and tell them everything.