Has anybody gone through this with stepkids?

Just keep reaching out but with no expectation of her. I have been the step kid and what happens to children of divorce, too many times, is the parents use them to pit them against each other and the needs of the child are ignored. She is put in a position of being afraid of losing her mothers love if she "betrays" her. So do not put even more pressure on this girl. Just be there for her when she needs something even if she rejects it. Some day she will wake up and realize you care, but this will take time and patience.