Thanks for the replies, everyone. It helps to hear the hard things as I know I have work to do. I don’t think I am ordinarily a fixer-upper, though I think I have to admit I got sucked into it on this one. Dad is a long standing high school teacher and very functional. I broke off the relationship after meeting the kids. We had a long conversation about the fact that if I came in and tried to make them ‘’better’ he would end up resenting me. The stupidest thing I did was believe him when he assurred me he was tough enough to cope with what was to come. The fact that his wife committed suicide on the back of delusional thoughts - she was convinced she had AIDS and had given it to the kids- made me feel comfortable with the explanation of how the kids got where they were. Ithink a big part though, is that I am conscious of having six children, who have begun having families of their own. So what right do I have to demand simplicity from my partner? Add to that, I work in the homeless/addiction sphere with some very difficult people..and I think I allowed some crossover between profession and life. Just because I CAN do it, doesn’t mean it’s a good idea. What amazes me most is the things we can ignore in the pursuit of happiness. I knew this would happen...yet here I am, still sooooo surprised.
I became someone I don’t have a lot of respect for yesterday. The SD has continually made a nasty little game out of ignoring my daughter and I..not all the time, but a lot of days..and then rushing in to say good morning, hug us etc once her father is present. So yesterday, when she said Good Morning yesterday, I just didn’t answer. Nor did I answer the next five attempts she made to engage. So she spent the day hiding in the study. So...since I have to acknowledge that perhaps I am losing the sanity battle, I am definitely moving out after work today. Everything is packed and ready. And its time to try to find normal again. I do plan to take the advice around counselling. I really appreciate the advice from everyone and definitely see a need to work through everything. I don’t ever want to repeat this mistake again.

I moved out last night. Not a complete move yet, as I am just staying with my eldest daughter while I figure out where too from here, however I thought it was important to go while I can. Fiancé did manage to thank me for everything I have done...and while I appreciate his words, it changes absolutely nothing about the situation. I'm really sad. Mostly because I know there is no hope whatsoever. I literally cannot stand SD and cannot stand being a part of the weirdness that is living in a house with two adults who spend their entire lives roaming around and never, ever, ever going out. Wait...I lie...I made them go out for a coffee together once. They lasted twenty minutes, and SD texted her father three times in those twenty minutes to 'update' him on her movements.
I have dodged the biggest of bullets, and I can't thank you all enough.