Q: I have two siblings — one bonus sister and one half-sister. We are very close; my dad is so close to my bonus sister that he walked her down the aisle at her wedding recently.

My dad's new girlfriend is throwing him a surprise party and has only invited me — not my bonus or half-sister. She said it was because my half-sister is away at college and my bonus sister is sober and alcohol will be served at the party.

My bonus sister has been sober for over four years and has no trouble being around alcohol, and my half-sister often comes home on the weekends. My sisters and I are furious. We want to like this girlfriend, but she continually does stuff like this. What's good ex-etiquette?

A: Certainly not picking and choosing which child you invite to a party. As a matter of fact, I can't think of a bigger ex-etiquette faux pas if your goal is to be accepted as "family."

You run the risk of offending when you pick and choose, and in this case, possibly alienate dad as well. How do you explain to a father that you arbitrarily chose to invite only one of his three children to his surprise birthday party? Very bad ex-etiquette.

I'm often asked about party invitations, particularly by people who have difficulty not taking sides after a breakup. They agonize about whom to invite, weighing each possible angle. "If I invite him, she will be mad — if I invite her, he won't come."

There's an easy answer. Although many feel the necessity to take sides after a breakup, more and more people are taking the stand to not choose — and that's good ex-etiquette. Of course if there was domestic violence or drugs or alcohol issues and you don't want to perpetuate a friendship, it's understandable taking a side, but if you wish to remain friends with both, just say so.

If one of the "exes" questions your allegiance, be honest and straightforward, and in a kind way simply tell them you don't want to take sides. Then, invite both, advise both that they are both invited, and let the exes make the decision to attend or not.

In your particular case, this would mean invite all the children. The sober adult child is fully capable of deciding if she wants to attend a party where alcohol is served, and the college-age adult child is fully capable of making the decision to go home at the time the party is held.

It is expected that all who attend will act like adults and not offend other guests with their breakup drama. If they don't think they can be civil, they should stay home.

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