Q: I have been trying to cultivate a more casual relationship with my husband's ex. We share the kids' time in seven-day blocks — a week with us and a week with her — and I figured a more casual interaction would help the cause. But yesterday she asked to borrow my favorite dress for an event we'll both attend. She knows I won't be wearing it — I just wore it to an occasion, so she wants to. I don't want to be that good of friends, but I'm afraid if I say no because it might upset my stepdaughter. She's so happy her mom and I get along. What's good ex-etiquette?

A: Well, you have the right idea in concept — reaching across that imaginary line in the sand to make it easier for the kids to go back and forth, but with establishing this sort of relationship comes the need for very strict boundaries. We usually don't formally talk about the boundaries we need when we make friends — you learn as you go and try to react sensitively as time progresses. It's necessary with this type of relationship.

First you have to ask yourself if you really do want to be friends or if your goal is just to be cordial so exchanges aren't drama. Most will say cordial — and that's probably the safest — because you are right, if there is a fallout of any sort it will affect your relationship with your bonus daughter. Plus, if there is a disagreement, can you imagine how that might put your bonus daughter in the middle? She will have to pick a side with repercussions any way she turns. (The real reason a close relationship with your stepchild's mother might not be a good idea.)

Now, in regards to borrowing a dress; ex-etiquette for parents rule No. 8 is, "Be honest and straightforward." Personally, I don't like to lend my clothes to people, nor do I like borrowing clothes. I'm always afraid I'll ruin them or they will, and I don't want the responsibility — and I would say exactly that. Tell her it's a personal boundary of yours. "And, by the way, maybe we should sit down and talk about some other things just to make sure we are on the same page." Your husband should always be present with any conversation that establishes house rules or affects his children.

Personally, I was very lucky and that may be why I'm open to relationships like you describe. My bonus kids' mother always kept the kids' welfare in the forefront (Ex-etiquette for Parents Rule No. 1, "Put the kids first.") We developed a very close relationship as the kids grew and they could depend on our consulting with each other and making decisions with their welfare in mind — but our boundaries were very well established. We assigned each other responsibilities — she was in charge of personal grooming, I was in charge of the more academic stuff — and realized that if we crossed the line we were both insulted.

We both knew that the kids' welfare is what counted and that was our bond. That was the concept on which the Bonus Families nonprofit organization was based and there are articles on the website with suggestions on how to establish a relationship with the other home when the kids go back and forth.

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