Agree with RoadRunner, this really has nothing to do with you, and you have no right to "confront" your sister with anything. She doesn't owe you insight into her romantical life, and if you try to pry, you're more likely to piss her off than anything. You should also be aware that if you cross your lines with your sister and/or intentionally/unintentionally are homophobic towards her, then at worst this can damage your sibling relationship over time. My relationship with my own sister at 22 is pretty severly damaged (for tons of reasons, but also lack of boundries and general respect). Trust me, you don't want that.

I think it's generally common to feel weird about finding out siblings being in love with others / having a relationship, especially at the age you're in -- but since you're linking this discomfort with her sexuality/being into girls, that's really something you need to work on yourself.