DREIST! Stepbrother harasses and gropes me

I was sitting on the loo unsuspectingly when suddenly my stepbrother stood in front of me without trousers and said he had to pee urgently. Oh man, can't he knock...and why is he walking around without trousers and even without pants? He knows I'm at home. And then he actually starts to fondle me, can you believe it? What if mum finds out?! What do you think, did he get away with it with me?