There is this girl that I really like. We both love each other as brother and sister. The only thing is, is that I like her as a girlfriend. She currently doesn't have a boyfriend, and we always talk on the phone and always hang out. How would I make the transition from loving each other as brother and sister, to loving each other as boyfriend/girlfriend.

You just tell her that you would like to see her more as a girlfriend than just as a friend like a brother and sister. Ask her out. Just keep up the communication with her as well as doing things with her. The rest should follow suit. It is important that you ask her what she would like out of your relationship with each other. Obviously, you don't have any problem with speaking with each other. Ask her. Develop the relationship depending on what she says and wants. You both need to be of one mind in order for the relationship to go beyond being just friends. Please communicate your feelings with her. Take the chance. That is how this is done.

She mite feel the same way and just hasn't told you.
Also, she mite have this weird "suddenly liking someone alot more as soon as they ask you out" thing that I have (no joke, when someone asks me out, I can't help but like them a lot more I don't know why its kind of weird)

she mite feel the exact same way and just hasnt told you.

This could be true!

Just ask her out! What's the worst thing that could happen? If you don't ask her then you'll never know how she really feels about you.

Your relationship seems pretty strong so id say go for it!! :) :)