Being honest with her always. You don't need to buy a girl nice things to get in. Be yourself as well. Don't ever pretend to be what you think she wants you to be. Surprise her with sweet gestures regularly and always be sincere.
If being yourself isn't working maybe she's not interested. But just keep being nice. Be honest. Be sincere. Be polite. Maybe she'll come around if she doesn't maybe one of the girls watching you being nice to this girl will.
bro if its my cuzzin then your already in her heart,shes young and i dont think she knows what she really wants in life yet ,im 42 and dont know what i want,seems as the girls i know want and do what ever they please,and if its my cuzzin tell her i could of used a txt msg to let me know she wouldnt be coming home and that shes safe,good luck porkchop.