I’m a high schooler with a crush on a girl, but she’s already dating my sister. Is this just teenage rebellion or something? I know it’s weird and I’ve heard it’s basically incest, but I don’t feel that way. Is it really incest, and should I leave this whole thing alone?

Crushing on someone who is dating your sister is not incest. On any level. Whoever told you that is wrong.

It is certainly uncomfortable and frustrating and annoying to crush on someone who you can’t pursue, and this probably won’t be the last time you feel like this. Learning self control around inappropriate crushes is a good skill that will serve you well throughout your entire life.

The best thing to do is, yes, to leave it alone. Crushes fade, but if you destroy your relationship with your sister over this, that will be much harder to repair. It is not wrong to have these feelings, but acting on them would likely hurt a lot of people. 

Try to minimize your time spent around this person. If she’s around to hang out with your sister, find somewhere else to be. Resist the urge to flirt. It’s generally impossible to just turn off feelings entirely, but don’t indulge or cultivate them.