Why is my cat attacking??

this morning my mom was in bed and my 7 year old cat was in there with her all morning rubbing on her and purring but wehn my 4 year old brother brought our kittens in there he set him on the bed and the kitty peeded and mom gets mad when that happens.....so she was yelling or somthing and the cat attacked her on her leg for no reason at all why does he do this?????btw this is not the first time hes did it......when the boys bother him he will hit at them but thats cause he is mad cause they r pestering them but please answer thanks!! 5***** stars