My brother who has autism was molested by a cousin

[debsqt06]

My brother who was diagnosed with autism has recently told my mother that when he was 8 years old our 6 year old or 7 year old cousin asked if she can come inside our house. He allowed her to enter the house and then she removed his shorts and also removed her clothes. After the incident our girl cousin called another 6-7 year old cousin and her younger sister. The 1st cousin taught our other girl cousin and the girl cousin tried to do that as well. My brother is already 27 years old now but it seems like sometimes he has this sexual urge and sometimes he will touch me or my sister. Once I felt it I will curse him and punch him with all my strength and my dad will just let him go out! Sometimes he will go excuse that he is dreaming. What is the right thing to do with our brother since he is special and his iq is really low and he doesn't want to be adviced or else he will throw tantrums and will suddenly become dangerous.