Okay I'll try to answer everyhing, so one thing after the other:
@Pret I am so sorry to hear this, this is actually the scenario I am afraid of and want to avoid 
@WhiteCat Well, I know that I need to improve this relationship to her, because they are a couple, if I want it or not. They weren't exactly nice towards me (If you want I can write a lengthy account of these holidays we went on) and I don't think that I have to take the blame for it and smile, say "it's fine" and pretend nothing ever happened. We are all adults now, and as well as I have to own up to what I did wrong, they need to see their shortcomings too. I don't blame her or my dad for the divorce, but for the way they treated me, and while my parents' relationship wasn't going well, I am pretty sure there are more respectful ways of breaking up almost 20 years of being married
@takeit So sorry to hear about your seperation, this sounds incredibly painful
@TooSassy my parents told us the truth. Something along the lines of "Your dad has another woman now and will not longer live with us"
@Dozer That's the model with us as well at the moment, but it results in very limited time with my father and awkwardness when she's mentioned like "I shouldn't have said that", which I would like to stop
@WiseUp I am so happy to hear that everything worked out after all. I agree with you that bitterness is not the way to go. I was a nightmare teen, but not towards my SM, I maybe saw her once or twice during my "bad phase", so on the recieving end of my troublemaking were my parents and teachers (If they ever read this, I am SO sorry), In my teenage years I was always very polite to her, but in a "cold" way, if you know what I mean, always overly correct, but as I said, I maybe saw her once or twice