As a step mom, I can relate. My husband and I also have custody of his daughter. Mom only comes around when its best for her ( been that way for a long time, even when she had custody). Therefore, my suggestion would be to talk with your husband and step-children and see how they feel on the situation. My step-daughter and I talk, as well as my husband, and if there is any event or activity that she would rather have just her mom go along, that is her decision. But if she says she wants me involved, oh well, mom will have to accept it. Her mom, when she does call, ALWAYS makes the comment that she is her mother. Well, we all know that and that doesnt change the fact that you are only her mom when it fits in the mom schedule and not my step-daughters life. Just let your step-children know that you love them and wants whats best for them, regardless of who it beside them, you or their mom or both of you. Best of luck!!