She has the same name as my sister and is my other sisters best friend

About six-seven months ago at the New Years Eve party this girl sat next to me while I was playing video games and started talking to me. I probably could have asked her out or something, but I was not sure if she was eighteen, I was twenty two, and she has the same name as one of my sisters and is my other sisters best friend. I now know she is almost nineteen, but I am now twenty three, she is mature for her age though. I'm really close to my family so I'm home every other day, while to say she has problems with her parents would be an understatement, so shes been at the house quite often. I've found myself feeling for her and I don't know what to do. Every other week I think I'll ask her out, but then she's hanging out with my sister or fighting with her parents. It's come to the point where I think I either need to forget all the reasons I should not ask her out, or I should forget about her. What should I do?

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