kids & ex wife

Terra
So my boyfriend is divorced. He has two kids with his ex wife. 
I've known the kids for years and have known my boyfriend for years. We have talked about getting married and having our own babies. 
The wife (like any ex wife) hates me...not a surprise. 
She bashes me to the kids and then when they see me that go off on these rants. I feel she's destroying these kids by talking about adult things in front of them but also talking bad about me and over people in front of them. 
The kids love me but when they are with my boyfriend and I for a weekend & then go home. The mom gets jealous when she finds out we had a good weekend and she starts telling the kids things to make them hate me. Now the oldest (11) is in therapy, but I'm not sure if it's helping. 
I always feel bad because my boyfriend gets the runt on the situation and gets stressed. I told him he needs to talk to the mom and tell her she needs to stop doing that to the kids just because she's mad and upset. I'm not blowing her being upset off, but I think any good parent shouldn't be talking bad about another person in front of her kids. She's teaching them bad habits that clearly are rubbing off on the 11 year old. 
I'm just kids ranting. As the girlfriend I can only say and do so much, but I would do anything for the man I love.