Boyfriends daughter

Hi guys! Me and my boyfriend have been together for almost 3 years. He has 2 daughters from 2 different women and he has a son with me. Both of his daughters are 13 while my son is a little over 1.

To give some context, my son is extremely attached to me and it takes him time to get use to people. He currently doesn’t want anyone to hold expect me.

I am having issues with only one of his daughters because of the way she treats my son/ her half brother. She constantly does stuff to aggregate and make my son upset and cry. My son doesn’t want her to hold him. And when she does he cries because he wants to come to me instead. She has make a couple of remarks saying “_____ I cant stand you” to my son. She makes fun of his appearance and she has also made remarks about his intelligence because he doesn’t speak that much yet.

Not only that, but she constantly gives my son a attitude when he doesn’t want to be near her. And she also gives a attitude to me and makes smart remarks to me for no reason. I been keeping quiet about this, but I’m getting to that point I am getting very frustrated with the way she treats him. I mentioned to their father/boyfriend about her attitude and stuff and he says she does that to everyone. He says he doesn’t even get on to her for it when she treats him the same way.

What should I do?