Q: My girlfriend and I moved in together four months ago. We have sons about the same age. They are nothing alike, and she compares them all the time and it makes it very uncomfortable. For example, her son is athletic; mine is artistic. She has said my son is weird because he doesn't want to play soccer. Weird! There's more, but in the name of keeping this short, what's good ex-etiquette?

A: I have found one of the most common reasons 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 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 personalities are different. Their histories are different.

Therefore, using a conventional family as your model doesn't work. The kids don't feel understood. They feel as if they can't do anything right. The parents are frustrated because "Your kid doesn't listen." Or, "Your kid is so spoiled!"

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 members.

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. It can be something as simple as, "Don't you think Justin needs a haircut? He looks a little shaggy." And the parent is thinking, "You think my kid looks shaggy?!"

The next time something comes up in the other child, the parent of the "shaggy" child will be sure to point it out. 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 families want to ensure success, they must learn to accept their family members' differences. That's good ex-etiquette.

Jann Blackstone is the founder of bonusfamilies.com.