Q: About a year ago I married a man with a lovely little 4-year-old girl. Since he and his ex share equal custody, I am with this child for a week at a time every other week. It has worked out well, and we adore each other. My husband and I wanted to take her to take pictures with Santa, and told her we would go this Saturday when she is with us. When her mother heard about it, she hit the ceiling. She said I had no right to take the child anywhere, and called my husband and really laid into him. Both of us are taking the child! What am I not seeing? What's good ex-etiquette?

A: First, asking, "What am I not seeing?" is another way of asking if you're contributing to the communication breakdown — and good for you for stepping away and trying to see things from her perspective. (Ex-etiquette for parents rule No. 7, "Use empathy when problem solving.") Truth is, there's not much she can do if the child is scheduled to be with dad, but her anger is an indicator that she doesn't like it one little bit that you're developing a close relationship with her daughter. Plus, doing things that they may have done in the past as a family really hurts, and she's probably reacting on pure emotion.

The every-other-week custody agreement is especially difficult for a parent who is used to being the primary caregiver. In this case, Mom gets divorced, her ex remarries and, all of a sudden, she doesn't see her child for a week at a time and another woman is taking care of her baby. It would not be surprising if Mom feels guilty for being divorced and worries that she can't be the mother that she wants to be. She has no control over the situation or what her child does when she's not in her care. Add that her daughter loves you, and you have one really scared mom. Rarely do we put our best foot forward when feeling vulnerable and insecure.

By the same token, you and your husband are probably rejoicing in finding each other. It's your first Christmas together and possibly the happiest time of your life. You're probably looking to establish holiday traditions with your new family, and taking Santa pictures seems like a logical choice. If you add children to the mix, it will probably become a ritual each year — a ritual that must include your bonus daughter because she is an active member of your family as well as an active member of Mom's family.

The thing that both homes must remember is life after a breakup does not have to be a tug of war. You can both be on the same side — the child's side. Although Mom may feel neglected and you may feel misunderstood, if you both use the child's best interest as the basis for decisionmaking, it won't matter with whom she takes Santa pictures — only that she gets to do it — possibly twice — and that's good ex-etiquette.

Blackstone is the author of "Ex-etiquette for Parents: Good Behavior After Divorce or Separation," and the founder of Bonus Families, bonusfamilies.com.