I'm so sorry for everything your brother has put you through, and continues to put you through, by you having to carry the burden of his abuse to you and your sister. I cannot imagine what you are going through.
Your brother raped your sister, when she was a child. He has continued to behave extremely sexually inappropriate towards you as an adult, even when you have asked him not to. He has no sexual boundaries. He has crossed the biggest sexual taboos, in sexually abusing a child, and sexually abusing a family member. He has perpetuated this abuse over years, when he was both a child and an adult. This is not a one off. This is a longstanding pattern of sexual abuse. The likelihood that he poses a sexual risk to others appears to be high. I am terrified for his children.
This is not a historic issue. His behaviour now is evidence that he continues to have no sexual boundaries and it has to presumed that he is therefore a continued sexual risk INCLUDING to children and family members.
By acting now and reporting him to the police you may either (a) Be able to protect his children from suffering further sexual abuse if he has abused them already or (b) prevent them from suffering sexual abuse.
Your brother needs an assessment by a professional to ascertain if he is safe to be around his children. And in the meantime, they need the safety and security of a home where sexual abuse is not occuring, or at risk of occuring. Their mum needs to know what's going on, and the children need to be given the chance to make any disclosures in a safe environment, if they have anything they need to disclose. Their mum will need help with knowing how to approach all of this.
This is not just about him facing justice for the historic harm he has perpetrated. It is about protecting children who cannot protect themselves, and breaking the cycle of sexual abuse.
Again, I am so sorry for what he has put you through and the burden you continue to carry as a result of his actions. I can't imagine the pain you're carrying. But his children's safefy has to be the priority. I'm so sorry.