Do you think my ex wife is using my brother to get custody of our kids?

My ex wife and I are in a custody battle over our two daughters ages 9 and 12 she is unstable she had threatened to kill herself several times during our marriage if I had left her she is also a recovered alcoholic and a drug addict. She been to rehab and has been clean for 3 years now but she still seems unstable we had file for divorce before she went to rehab. We had a good routine going when I had custody of the kids she had visitation she would visit them at my house supervised I will be working but still keep an eye on her just found out that she's been dating my brother my bro told me this I was angry at first but I warned him what she could be capable of I am not supportive of the relationship then all of the sudden my brother and ex invited the entire family over for dinner and announced that they were engaged!!!! My brother's wife died a year ago and she is taking advantage of him. They have been dating for a month!!!!! I think she is playing him and now she wants custody of our kids!!! So I told my brother that I don't want my kids having a uncle and a stepfather my brother is in denial and she is manipulating him what should I do?