Well I was taking a shower, just minding my own business...?

when out of nowhere my twin sister came into the bathroom unannounced and snuck in the back of the shower while i was washing my hair and started putting soap on my back. wtf. She does stuff like this a lot, and i've tried talking to her and all she says is "oh come on, we are brother and sister, whats the big deal" ... the big deal is we arent 7 anymore. we are 19 and its a little weird to me. Call me crazy.