I could see the extended family concept working (and I certainly agree that finished is a better term than failed), but it didn't work in my case. For someone who is divorcing, finished means you've completed a chapter in your life, and that chapter no longer works for you. Failed somehow implies that you've wasted the time you were married (or something). This might be a positive thing in the end - you can't tell that at this point(and it sounds like you've accepted that you want to maintain a relationship with your step-dad), but the process (including the discussions) sucks.

I don't want to go into too many personal details, but I have a parent who considered divorcing a step-parent. This parent chose not to, but not before dragging me (and my grandmother) through the awful details of the marriage (no holes barred). I would encourage you to be firm in what you will and won't discuss. My parents (divorced when I was very young) never stopped openly sniping at each other in conversations with me (they were polite to each other, and then slammed the other to me).

They don't owe you an explanation, and you really may not want to hear one (at least at this point). I would simply explain that they've made a decision, you respect their decision, but that you really don't want to discuss it with them. Sometimes families need that explained. Holidays can be hard. Too many people and too little time. I'm still working on finding that balance (though this year, my decision was to stay home and take time for myself). But, there is a balance, and you don't owe anything to anyone. Take care of yourself (and make sure your grandmother is taken care of - I can't imagine how she feels living there!?).

Good luck to you!

CA