Anonymous wrote:OP here. So we spent the weekend with them in the same house. I observed them. Didn't see anything sexual. But he was using her for his entertainment as a toy. He would grab her and drag her when she said "no".
So I intervened when he did that. I told him to stop. "Larlo, if your sister said no, you must stop. No means no. Stop dragging your sister." Or "You have to respect your sister when she says no." I said that in a loud, harsh voice and he would stop immediately. He didn't like it, but surprisingly neither did his mother. She objected and I told her the girl will grow up knowing her "no" doesn't matter and that he overpowers her. Mom denied it.
Also, told his mother that the girl needs a separate bedroom at this age, it's not appropriate. She said the girl was getting a separate room.
Also, told the girl that when she says no her brother has to respect it and if he doesn't she needs to tell adults.
Overall, the boy was obnoxious and I don't want to spend any more weekends with him around.
Thank you for updating. What a sad situation. You did a good job, OP.