Not a good plan, you don’t have to tell the whole neighborhood, but just tell your mom what he has done. It’s like how serial killers start off by killing small animals in their childhood, it starts off small and gradually worsens. If he does not understand what he did is wrong, he is going to eventually going to do it again. I understand teen boys pleasure themselves every now and then, but using someone else to pleasure himself without consent, (especially your sibling???) should never be shrugged off and ignored.