It's very diffcult at first but very rewarding once you establish trust with them. Trust me, she'll come around... hopefully sooner than later. My FI has 3 (yes 3) daughters ranging in ages from 7 to 17 ! It took the middle one quite some time to warm up to me- Years infact. Shes finally to the point now that she can actually laugh and tell me "I hated you so much when you first started dating my dad"! Patience is key for sure; especially at her age. Little girls are very protective over their daddies. Once she figures out that your not trying to take her dad away then you two can be great friends! Good Luck!