Provided all of you who would be parenting and providing for her should agree, it seems like a fine idea to me. I was lucky to become a father when I got married, not the usual way things go, and that's worked for the best. I've found other people too, older, who could have used someone so much sooner in their lives. It's an important void you'd fill for her. If there's one thing I've seen, son, it's a huge lack of people who care enough about doing that when people are young, and they grow up to suffer for it.
I don't much know about the condition she has,I take it something that's not known about where I'm from, but it doesn't sound like something that makes a difference to you, when it comes to this kid. What it does sound like is that she's already probably becoming yours at heart.
Just something to think about since you were asking opinions.