Any advice from stepmoms or stepdaughts...

My dh and I just got married this Sept.
3 months before the wedding we met his daughter for the 1st time (she's 15)
He had told me that he believes to have a daughter out there when we first started dating...Her grandmother moved away when she was a baby and raised her...her mother moved away too in the other direction. We did try to find her for a while and then out of no where he got a call from an old friend saying she was here...long story short..everything went great and has been going great...we even flew her her for the week of the wedding...I actually found a dress 2 days before the wedding to match the bridesmaids. She is now coming for a week around christmas.
Her biological mother has never really been part of her life. she seen her 2x in 12 yrs. She has had every opt to be part of her life and choose not to. The past 2yrs she has been in contact with her they do talk on the phone but since she is now part of our life her mother is really giving her a hard time. She actually told her not to bother calling her whille she was here and she want to see pictures but she has to cut everyone else out..she doesnt want to see anyone else! Obviously she has some issues, she is now making her feel guilty for coming her for christmas and not there...the past few years she would tell her she'll fly her out to see her and as the date approach came up with some excuse no to(I never imagined someone could be so selfish...you would think she would be happy that she now has a dad who loves her and want to be involved (he always has wanted to but not making excuses but he was very young when she was born but as he got old it haunted him...her birthday was alway very hard for him)..her mother on the other hand had every chance and choose not to be part of her life..
we talk to her almost everynight now she is part of our family!

My problem is...I'm considered the "cool stepmom" that I can say anything to...which is great but in the same breath but can you remember when you were that age? All the issues would you tell your mom? Im happy that she feels like she can talk to me but man Im sure its hard enough to have a 15yr old with 15 years leading upto these issues its alot when its basicly over night. I know its hard for him too...esspecially being a man!

Well I am new to this site...and its been years since I've been on a chat board...I forgot how nice it was to be able to vent/babble and hear everyones advice and opinions