Is it normal my best friend feels like a sister?

My best friend is a girl I've known really well for three years. She's shared so many intimate details of her dating life, even sex life; she's vented about frustrations, anxiety, and other usual things best friends might share with only those closest.

We're not perfect, because we can have war of words at times. She tends to point out my every flaw as soon as I make any small error in doing something, according to her standards. Now, this used to upset me, the solace I find in her reactions like that, is that our intimacy as friends is so tight, I genuinely believe she's simply showing "tough love," as a way to help me be better at certain social activities and mannerisms.

Any time something traumatic happens in either of our lives, we release that stress through deeply sincere conversations that we keep between us. I feel she's trustworthy and I know she trusts me to the utmost degree, it seems. I cry when there's that one thing that it.takes a bit.for her to open up about though, because she never holds much back and if she does, it's always real depressive and I get upset if she's upset.

Bottom line, I help her. She helps me. In virtually everything life throws at us. Normally, I always thought girls have a girl to listen to them vent. And guys talk to other guys. Why are we so close, almost.like best friends? Is it normal for a guy to have a female.best friend, whom he isn't romantically inclined to, and is it normal I think she's practically like my sister?