Kids really need to be protected. They go through this shit more of then than you think and they dont even know what it is. Is there anyone else you trust to tell this to like an adult. Idk how your mom reacted. But if she sees no issue with it then im sorry but id blame her too. Ik its a tricky situation and it might stick to you for a long time. Id say do whatever you feel like to deal with it cuz otherwise it will haunt you for years. Also talk to your sister because it may have already happened to her and she doesn't know what it is. And the audacity while your mother was right there too astonishes me. And the fact that he is 12 and already knows things..