I am new to this, I found this site just looking for other stepparents that could give any advice. I'm in therapy but hearing opinions of others going through this might help. 
So a little background, we have SD19, SS15 and SS13. I have known them for 10 years married to DH for 4. BM has always been high conflict and wasn't nice to the kids growing up. She tried keeping them from their dad, but wouldn't take care of them (showers, school, sports). When SD was 16 she moved in with us full time and the boys followed shortly after. It was wonderful! We were a fairly happy family, vacations, homework was done they all had friends and seemed to be thriving. When SD was 18 she got a significant other that we said couldn't live with us. We thought it was inappropriate for the boys. she moved out and has been moved out ever since. That being said she would still come see us and it was fine, until a few months ago. She just stopped coming around. We can't get her to come even to dinner. The only times she will are if her brothers beg her or if we are buying her something/she wants something. She will however go to her moms a few times a week and basically treat her like a friend. 
I have to say it feel awful! We were so close to her and had such a strong bond I never thought she would start ditching us. My therapist says she is probably going through something, trying to grow without us since we are "the parents" and her bm is the "friend". But it feels awful, I just want to feel better but don't know how. Any advice would be wonderful.