Q: My girlfriend and I moved in together about 4 months ago. We both have sons about the same age. They are nothing alike and it’s really difficult for her — and me — because she compares the boys all the time and it makes it very uncomfortable. For example, her son is very athletic, mine is not. He prefers to draw and she has out and out said my son was weird because he doesn’t want to play soccer! Weird! What’s good ex-etiquette?

A: One of the most common reasons why many combined families don’t make it is that there is very little acceptance of differences. Most parents attempt to combine their families based on what they believe to be a conventional two-parent family mold where the children are brought up together from the beginning and have many of the same likes and dislikes. They have many of the same memories.

Bonus families are nothing like that. Their memories are created later. Their histories, what makes them tick, are different.

Therefore, using a conventional family as your model just doesn’t work. The kids don’t feel understood. The parents are frustrated because “Your kid doesn’t listen.”  Or, “You know it’s a little weird that your kid doesn’t like soccer. What kid doesn’t like soccer?”

Ex-etiquette for Parents rule #3 is, “Don’t badmouth.” That rule isn’t just for co-parents badmouthing each other, it’s a guide for talking about any bonus family member.

Just as a child will personalize it when one of their parents badmouths the other, parents personalize it when their partner says something derogatory about their child.  Once that starts, the family breaks into factions — your kid and you against my kid and me. That’s difficult to repair. The damage is very quickly done.

If bonus family members want to ensure bonus family success, each family member must learn to accept their family member’s differences. That means you don’t make comments like, “Your kid is a little weird.” Instead, you support his artistic abilities.

Acceptance. That’s good ex-etiquette.

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