Please somebody tell me what to do. My stbex has struck up a relationship/friendship with my ex sisiter in law! They are now in cohootes together. She tells her children not to tell my brother things and uses them greatly. I found out last weekend that he had my childrens cousins (her children) and her stay over at his house that my children have only stayed at twice. Tonight he spoke to my daughter and asked her if next weekend they want to go to the park with her and their cousins but not to tell me! My daughter said no should would rather only go if my brother was going and not with her but then got really upset when she told me because she didn't want her dad to get cross with her for saying. He then proceeded to torment me about going to court and getting more access all of which i have never denied he has just never bothered. I have told him to go through the courts if he can't be man enough to sort things out with me, I am not being unreasonable. I am devestated by how twisted he has become and I am sure it has gotten worse since she has been on the scene. i don't want her anywhere near my children help I don't know what to do.

Unfortunately, unless she poses a threat to your children there isn't really very much that you can do.
Sounds like your daughter isn't very happy with the situation so she could discourage contact with her anyway. Just reassure your daughter that you won't tell her dad things that she tells you - and don't tell him. That way she will know that she doesn't ever have to hide things from you and you will allways know what is happening.
Don't do anything re court - unless he actually does go through with his threat. Don't let him "torment" you with it. If he does, there's nothing you can do about it so don't worry until it happens. Don't give him power by allowing him to hold things over you, don't react to his new partner, let your daughter seem him and her if she wants to, but try and remain detached about the situation. Good luck !