Divorce and step-parents?

My husband and I are going through a rough time right now, we always argue about the same 2 things. One of those is my step daughter. She is in her mid 20's and I have issues with her coming to our house. She never pre arranges or asks if it's ok to come just sends a text message saying she will be here in an hr. She then is here and it's tense as I don't like her and can find no endearing qualities. Up until this week end I got on really well with my step son. While she is here it's all about her and you can't get the conversation off of her...even though it was a bit of time to talk about a recent family engagement. She tries to subtly get money out of her dad...ie I have no money to go out this week end and I really want to go out...luckily he doesn't do subtle, he never picks up on it. We don't have money to give her and I wouldn't be happy if he did...I know it's his daughter but she works and should be supporting herself. My main question is this...if we do divorce what should I do about my son. This man has helped me raise him since he was a baby. My son calls him Daddy and always has, it's the only daddy he has ever known. I don't want the step daughter anywhere near my son...she is disrespectful to me and he then starts it. Do I sacrifice my son's relationship with his "Daddy" or what? Yes I should be the bigger person and 'get over it' and her treatment of me, but it's hard when all she makes comments about her family when her parents were still together all the time. It does take a toll on you after a while and this has been going on for over 12 yrs...I can't do it anymore. If she wasn't in the picture we wouldn't have the problem.