Step mum advice please?

hi,i have a 14 year old stepdaughter who as lived with me since she was 9 years old.since she came to live with me her real mum and her mums side of the family as not even tried to contact her all this time.now last week my stepdaughter told me her mum had added her on facebook,asking her questions also asking if she wants to come visit her.i feel that she should be talking to myself or my husband instead of asking her direct.she as blocked myself and my husband on facebook so we cannot talk to her.in the first place when my hubby divorced her as she was having a affair she stopped my hubby from seeing his child for approx 5 years,it was through the grandma that we had contact with my stepdaughter.after she visited me and my husband a few times she told me that her mums partner had tried it on with her,after involving the police,we found out it was untrue.this is the reason for her mum not wanting anything to do with her.her mum and grandma always said that we put her up to saying that when this was nonsense.now we are stuck because we feel that she is trying to get her daughter back to live with her and are afraid what will happen.please advise thank you,