Hi, I'd like to help you with your question.
This sounds like a very difficult situation. You are right, the ex should not play such an active role in your husband's life especially since there are no children still at home to care for. The most you should see of her is at events related to their children.
One of the big factors missing in this situation is what is your husband doing about this? This is a situation he helped create and he should be dealing with it. It is his ex wife and his family that is causing the issue. Since you are his wife, your feelings should come first about the situation. That is not to say you can be unfair and demand to never see the ex wife again. That is unreasonable. But you do have the right to be respected, not only by your husband, but his family too. If he is showing that he doesn't put you first, before he ex wife, then his family will not feel the need to do so either.
Speak with your husband about how you feel. Be direct but not hostile or angry. Let him know that this situation is difficult for you and that you would appreciate more respect from him and his family. Encourage him to talk with his family and tell them he is upset by how you are treated. He should tell them that he is unhappy about the ex wife being so involved. Also, he should have a plan on how much involvement his ex should be having at family events. For example, she should be invited to only events involving the children. And if his family wants to see her, let them have contact on their own such as inviting her to lunch or something similar.
If you feel that this issue is starting to interfere with your relationship and your husband will not address the problem with you, see a counselor together. If he won't go, go by yourself. You need support with this problem and you may want to explore why your husband will not respect your wishes in this matter.
There is also a book that may help called Ex-Wives and Ex-Lives: Survival Guide for the Next Wife by Paula J. Egner.You can find this book at Amazon.com or your local library may have a copy.
I hope this has helped you,