My husband and I share 7 children and 2 ex-spouses. We probably encountered more than you'll ever face, hopefully. Maybe I can make suggestions from our last 20 years that may or may not help. Either way, know you're not alone and someone cares enough to write.
You have several choices. You must walk in the light and ability that you feel comfortable with. I will start with what I believe to be the best recourse, but you must decide which to access.
1. "Love Never fails". It sounds like the ex-wife is lonely, bitter and emotionally wounded from experiences prior to her failed marriage. IF possible, invite her to a womens' week-end retreat where she may be able to find spiritual help. Who knows, you two may become friends in the process, and she can turn the need for attention from your husband to you.
(My ex-husband was invited to every Christmas/birthday, etc. and stayed in our home until all the children left on their own. It was uncomfortable at times for me, but the children appreciated the effort.) (We tried with the other ex several years in a row, but there was more there and it eventually ended on its won.)
2. PRAY for her every day to find help, make new relationships and discover who she is; pray for freedom from the traps the enemy laid for her and emotional healing.
3. Ignore the situation, and decide not to let it affect you or your relationship with your husband. Understand that we all make mistakes, and pay a price for the mistakes. Unfortunately, when children are involved, those mistakes last a life-time.
If you'd like prayer, our information is at ICServants.org