Hi Linz,
Heartfelt commiserations.... When I first got to know my new lady, Linda, I had also to meet her grown up kids and, as a new twist to an old tale, she took me home to meet her kids. Her two sons were OK, though I remember a delightful incident the next New Year, when both of them were rather drunk, and they took me to one side and sternly, if hazily, told me to be good to their mum or they'd do dreadful things to me. We now get on really well.
But Linda's daughter was another situation entirely. Living at home at the time, my later-to-become stepdaughter was in the throes of a relationship break-up, and life in the house was fraught - on a good day! Apparently in an effort to get me to leave, so she could have her mum to herself, presumably, she once described me as a ****ing axe-murderer (which I've never been! I've blunted and broken a few tools, but never murdered an axe in my life!). She was frankly impossible for a couple of years, and tested the relationship Linda and I have to the absolute limit. We nearly parted; very, very nearly; but I was incredibly reluctant to let a petulant daughter rule my or her mother's life. So I hung on in there, often losing sleep and sometimes wondering why I bothered....
Well, it all blew over, after about three years. The daughter is now an absolute gem. She respects me for hanging on in there when she was so diffficult, recognising she was, and that it was deliberate, and she chats happily and now asks my opinion and advice quite often. Having now moved out - which was a significant evolution for her; she went to university, which was another milestone; she even rings to speak to me, rather than her mum. It feels odd, but delightful, and I'm proud of my stepdaughter, now. We do look back on the troubled times and laugh now, when at the time we cried. But 'love won through', despite my fears it couldn't.
I really hope your SD evolves into a rational, responsible, agreeable human being in the very near future. I now have no regrets, and I'm really glad I stuck with it.
But, before you ask, once was enough.... I'm not offering to take on your challenge...! But all the best with it / her.
John