This is so embarrassing but I need help

Ok I’m posting this anonymous for obvious reasons.

My brother in law (adopted) loves to play with women’s feet and it gives him boners. He’s very small, about 4-5 years old when it started.

At first I thought he was just playing, like pretending to paint my nails or something silly but he goes way too far. Forcibly removing my sandals, rubbing my feet, (even though I continuously pull away and tell him that’s enough). He rubs his face against my feet and kisses them until he gets an erection. It sickens me and I try to stay away from me because it makes me seriously uncomfortable. I told my mother in law to make him stop but she just laughs and so does my husband. I feel like it’s pedophilic and I don’t know why he does this all the time.

It has gotten to the point I hardly want to go over to my mother in laws house anymore.

Today as we were leaving he came over and sat next to me on the couch and leaned his head on my shoulder and I thought “oh how sweet”, that’s until I felt him to try to put his hand down the back of my pants. I said “Evan stop, don’t do that, that’s not appropriate” but he just laughed and ran off.

I can’t wear skirts or dresses over there anymore because he will try to run his hands up and down my legs the entire time.

My MIL is way too lax with his behavior and I once saw him put his hands on her breasts and begin to massage them about, mushing them around. He did this for like a full minute before she noticed and told him to “quit it” and he laughed and ran off to play,

He seriously creeps me out and I know he’s a little boy but he’s so Pervy!

My husband thinks he is a little skeez too but his mom doesn’t listen to me or to him about our concerns.

Help!

EDIT: he was adopted as a little baby. My MIL is the only mother he has ever had. His parents were illegal immigrants and were deported when he was like 10 months old. Because he was born in the US he was permitted to stay and his parents thought he would have a better life in the US.

He’s 11 right now.