I wouldn't force a friendship on the girl at all. Odds are she told her mom about the FB request and the mom probably had some choice words about you and like someone else mentioned, she feels loyalty to her mother. Besides, you never mentioned how or why her parents marriage ended..did he cheat? Did he up and leave the family? The girl may not have a whole lot of respect for her father, if thats the case then don't hold your breath on her wanting to be besties with you anytime soon. I also find it surprising that given the fact you and your bf are talking marriage that you've not actually at least had dinner with the girl. The man doesn't seem real concerned about his own childs life, don't expect anything better from him. Lord knows if he treats his own kids like this he'll never treat a woman any better. That I know.
So, so you think you can tell Heaven from Hell,
blue skies from pain.
Can you tell a green field from a cold steel rail?
A smile from a veil?
Do you think you can tell?
And did they get you to trade your heroes for ghosts?
Hot ashes for trees?
Hot air for a cool breeze?
Cold comfort for change?
And did you exchange a walk on part in the war for a lead role in a cage?