She most likely feels close to you, but whether it's romantic or not is not certain. You'll have to ask. I would suggest doing it as low pressure as possible, keeping it more like "what if, someday" talk to feel her out....the most touchy part is that she is not single. INFPs usually have a very strong sense of loyalty, so you want to respect that. What do you know about her relationship?
As for "normal" behavior....some of my best friends are guys, and I might behave similarly with them at times; but there are some indicators that she may like you as more than just a friend/brother figure, or be open to the possibility in the future if she were single again.