NAH. It's your choice whether to forgive her or not. However, I highly doubt she was now realizing she was wrong. I wanna talk about this:

I don't think I can, though? For one thing, like... she's just now realizing she was wrong? After she literally watched me get abused for a year then kicked out? Plus, I just can't reconcile the fact that she's still close with our parents, who are sending her to a great school and giving her such a wonderful life. It's just not fair.

I don't wanna put any words into the story, but you gotta understand that maybe it isn't all that seems to be. I've seen a lot of people on reddit in abusive households, stay on their parents good side for benefits of a better future (Going to college for example). Especially those in cultural or religious abusive households.

More than likely she had doubts for awhile, and it just took her awhile to reach out out of fear. She does seem very genuine in her text (Unless it's paraphrasing, then I don't know). Maybe her parents had threatened her to not contact you or she feared you would reject her. There could be a bunch of reasons she waited until she was out of the house. Unless we just don't know, but I would still say NAH because she was just a kid, under the influence of both of your parents.