I was sexually abused by our family GP for over a decade. My mother knew, and said I was lucky because he was gorgeous. She did nothing to stop it, and flirted with him at every meeting, and tried to have an affair with him, and until I stopped talking to her a few years ago would refer to it all as if it were an exciting and fun piece of gossip. Yeah. Right.
I am now 54: I reported him to the police about three years ago and they were brilliant: so kind and compassionate and very careful to do everything at a pace I could cope with.
I had an initial meeting with them, in which we discussed the outline of what had happened; then I gave a three-hour video interview. I named other victims, and one of them also gave a long video interview. Then the police went to arrest him, and found that he'd very recently died, the bastard. So that was that.
If he'd been alive he would have been taken to the police station and questioned. They told me he might not make it to court, as he would have been very elderly; but even if the CPS felt court wasn't appropriate, he would have been pressed to accept a caution which (I think) would have placed him on the sexual offender list, which would have been good.
You can't do this anonymously and if it does go to court it would be prosecuted under the laws in force at the time of the abuse, which reduces the sentences which can be given. But you don't have to go to court, necessarily, as your video statement will be used instead of giving evidence.
If your abuser is a member of your family then yes, reporting them to the police does have the potential to split your family. But since my parents were out of my life (over three years now) I've found everything a lot, lot better. It might not be the negative you're worried it could be.
My only regrets are that I didn't report him to the police sooner, so he could have faced justice before he died; and that I put up with my parents' abusive nastiness for so very, very long.