Q. My boyfriend and I moved in together last April. We dated for a year prior to deciding to combine our families. I have two daughters, 7 and 16. He has a 12- and 17-year-old son; the oldest is planning to go to college in Hawaii after graduation. Recently I noticed the vibe is a little different between our oldest children and I am concerned they may be attracted to each other. What do we do? What’s good ex-etiquette?

A. Ironically, I find parents often categorize kids in two groups. One, my kids. Two, all other kids. And all other kids stretch the truth, sneak out at night, drink alcohol, experiment with drugs and have physical attractions. So they don’t consider that their children, about the same age, may be attracted to each other, and move in together without the proper checks and balances in place.

What are some of the pitfalls they may run into? Leaving the kids home alone without adult supervision and the obvious problems that may lead to, the impact their relationship and a possible breakup might have on the dynamic of the bonus family, setting an example for the younger children, just to name a few.

What are the alternatives? The parents come to agreement to what is acceptable behavior, then each parent sit down with their biological child and explain the rules and the consequences. Make sure the teen is allowed to consider what the impact of their relationship might have on each family member and the family as a whole. After that is done, the parents sit down together with the teenagers and explain the house rules and what is expected. Should this have been anticipated prior to moving in together? Of course.

Now, the elephant in the room—birth control. Please don’t write me about condoning underage sex. I am not. Because you have an open conversation with your child does not mean you condone their having sex. Make that clear. Make the house rules clear and don’t set them up for failure. 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.