Why does 18 year old stepdaughter always put her family last?

My husband and i raised my stepdaughter since she was 5. We gave her a good spoiled life even though we were strict about grades and boys. She HATED our rules and fought with me about sneaking out, skipping school, and the whole birth control issues every mother has. Her biological mother never took care of her and chose drugs and younger men over her and her sister. For 7 years her mother didnt speak to her. Since she has turned 18 she has a new boyfriend and moved in with him after 4 weeks and his stepdad supports them on every level. She doesnt come around, we have to practically beg her. She constantly breaks her younger siblings hearts and they dont understand. She ran back to the bio mom upon turning 18 and then soon realized the mom still hasnt changed. If one of is is in hospital she won't even check on us. I still provide for her in some ways and im always asked why. She treats me bad and talks to me like a pos. We used to be really close and she changed. Everything is about her boyfriend and his family. She wont call or anything. We didnt even do anything wrong. Now she gets drunk all the time and acts really bad. She can be a sweet girl and friends have seen her mistreat us and say she will grow up someday but when? She gets mad over the least little thing and will disown us for months. Then she tells family we mistreat her and it gets back to us. We have no idea why she acts this way? She even blew college off for this boy. He has a stepdad w money and thinks they can live happily ever after off him. We know our daughter is not in love but only secure. My husband doesnt even want her around anymore bc of everything she has done and he is heartbroken. I was always known as mom to her and now she treats me like total crap and i take it. But i cant do this anymore. I offered to pay counseling and she refuses, asked her to come over so we can all talk and she is too busy. What is wrong with this girl??? no i dont believe she is on drugs. She went from size 2 to 8 in 6 months time. Please tell us whats best? Our hearts are broken .