My cats son keeps trying to mate with her?

My cat had kittens but we kept 2 and it's brother is staying over for a while an they kept bitting his mothers kneck and trying to mate and it's mum smudge was doing some sort of stepping dance with her back feet and so we new they were in heat buts so disgusting smudge know that's her sons so we keep them in separate rooms and 1 of her sons called sprint always meow in a funny way now no stop and yesterday they acidently got in the same room and we heard a pound moanings sound like almonds getting hurt it almost sounded like a person out side was shouting and when we looked sprint was on top of her but she wanted to get away and wen he went out side he keeps trying to getting like I put at the front and he goes to the back over again and his eyes were wide like he had done something i.e. Rape and I was really upset coz thief like familie and family shouldn't be mating with each other and were not sure if she's pregnant or putting on some wait because she is very skinny when we got her and she's had 4 litters 2 with us but when we got her she was already pregnant but we didn't know. Oh we haven't got them done because it expensive and but will it stop