Look, we don't all agree about the background here but we do all agree that this situation sounds v tricky and that OP should back off or at least tread with utmost caution. So in practical terms, we're all on the same page.
I hestitate to mention my dear old Maw again, I see she was discussed with vigour up there a bit. Ok, well I asked for it and I see that. But I should mention that my mums temptress days are a long way behind her, she's worn elasticated slacks for years know and hasn't stolen a husband in a very long time.
I never said that she was blissful with my stepdad. They are just normal retired old farts bickering about everything, same as everybody else's parents.
I suppose what I want to say is that while I know many of the affair cliches are true (have seen with my own eyes in friends lives etc), often they aren't true. My mum can talk to my SD in a way she never could to my dad. She is massively more suited to my SD than my dad. Not all relationships are the same. Look at your own relationship, has the bond gone because of housework and domestic drudgery? Most of us can get past that.
It's not black and white. I spent my childhood wondering why women laughed so much. Now I know its because my awesomely handsome dad was flirting with them. He has been happily (but boringly, normally) married GP my lovely stepmum since Wham were in the charts. He can talk to her in a way he never could to my mum.
I know most people don't go on to be best friends with their ex and their new partner. But it can happen. I'm glad it did in my family. I have four relatively happy parents instead of two pissed off ones.