The biggest problem is that she was there before you were! If you start a relationship when young, free and single, then it is you and DH as the main couple and all else flows from that. When someone else has a 20yr head start on you for a close relationship with your dp, it gets complicated. Very complicated.... Throw in wider family taking sides and not wanting/absolutely bloody refusing to integrate a new woman into the family, and you need all the wisdom you can take onboard.
As a new stepmum, you are not just finding your feet with a new man. You are simultaneously trying to keep the entire family sweet. Tall order. Every move you make is scrutinized, and you might as well put cameras in all common rooms now because nothing will be private from then on, and at least you'll have evidence later!
Do I sound cynical? Anyway, if you move in with dp, an adult daughter is already the lady of the house, so tricky, tricky, tricky. If dp moves in with you, you are lady of the house, but there's still a tussle for the kitchen and protests at any rules/restrictions, however reasonable.
Adult children may have expectations of financial/housing support that depend entirely on your income and capital. That can come as a bit of shock.