My now husband's daughter was a nightmare. She hadn't even met me and was emailing me abuse, texting me abuse and calling my home. She would do her damndest to spoil our relationship and made it very plain that she would try everything to split us up. For while my husband tried to make excuses but the day we were invited to a BBQ and she followed us and hurled abuse over the fence sealed her fate I'm afraid.
I know this may be difficult and, believe me, I had one hell of a time. But I decided to not rise to the occasion or let it come between me and my husband. He finally saw her for what she was and told her she was a spoilt brat and he would welcome her back when she grew up.
Three years on, and after an apology for her behaviour, when she realised he wasn't joking I'm the best thing since sliced bread LOL
If he's worth it try to keep a lid on the anger that will almost certainly arise - but I agree with other comments - don't try and be her friend. See if Dad can find out what her real issue is with you. My step daughter didn't have an issue with me per se - she didn't know me - she just wanted Mum and Dad together.