Honestly, I wouldn't bother with him. If he is showing more interest in someone else, then that is who he wants. He's basically saying if he can't get what he wants, then he MIGHT settle for you. You don't want someone who is settling for you, you want someone who wants you.

Trust me, there is someone out there who wants you for who you are & not who they wish you were. Just make sure that you want that person for who they are & not who you want them to be as well...

As for the whole "like a sister" to me thing, lots of guys use that one & they all mean something different by it. My sister for instance is a control freak, so me & her don't get along so well. The next guy may have a wonderfully communicable relationship with his sister. So it could mean anything.

But take my advice & do not cave in to him if he approaches you like he wants you. He doesn't. At least not how you want him.