Inappropriate mother son relationship?

My sister has this friend who has a 13 year old. Me and her talk but I wouldn't consider us friends. I don't really hear her son talk much because every time someone tries to speak to him she talks for him. I never really see him leave his mom's side and it doesn't seem like he doesn't want to. It's like she won't let him. He tried to go sit with my daughter and her friends and his mom pulled him and told him to stay by her. She also always constantly puts her hand between his legs. She says it's to make him feel better. Idek what that means. I think that's just really inappropriate. I did invite her kid to spend the night with my son for his birthday but she said no and that he doesn't leave home plus he still wets the bed so she doesn't want that to burden me. I told her it doesn't and she still was like no. Some may wonder if he has developmental issue but I wouldn't know because it seems like he's not allowed to speak. And the very few times he does speak she has to be holding his arm and I always see her squeezing his arm while he's talking. When she does that he stops talking or changes what he was talking about. She says he also doesn't have his own room. He sleeps in the bed with her. I get depending on certain things that can be okay but when she talks about it her son just looks unhappy and looks away. I just feel like their relationship is inappropriate...