Lots of straight girls watch lesbian porn, women's porn habits are pretty complicated and don't match our sexual behaviour/actual desires. Pretty much, we can get aroused by sexual imagery because of a function our body has. It seems we're hyper prepared to respond to sexual situations with common signs of arousal to avoid injury from sexual assault. It's pretty dark...but a very depressing truth.

Watching porn isn't something we're evolved to do. But consider this response a loophole. I guarantee you could get into any kind of porn if you exposed yourself to it long enough.

It's hard to say what your sexuality is, but please don't stress this much over things. The only surefire way to know is when you find yourself crushing on a girl.

I'm inclined to believe you might be attracted to the same sex because you got so defensive with your sister. This is pretty common, it's so easy to internalise all the shit people say about lesbians and fight against it because...some part of us suspects, or even something deeply unconscious.