If it was your child would you want to know even if it was your own brother? Yes, I'd be much more protective of my daughter than any relative.
Really how would it make you feel? Awful.
Would you believe them? I'd believe my daughter.
Would you hate them or blame them? I think I'd blame myself.
Would you be okay? Of course not. Are you OK? You think you can 'keep' everything OK? It's too late for that. You will be OK when this is all out in the open and then over.
Don't assume that they will call the police. They may be too ashamed of your uncle.
There's a saying 'the cat is out of the bag.' You can't avoid this anymore. Your mother is more and more sure of something going on. Just tell her.