Your sister has a key to her own room

And she comes back from her run, horny as hell from seeing other fuckable young people, to witness her boyfriend laying a gorgeously slutty, clearly well fucked and visibly formerly virgin Chinese teen on her bed, her little cunt trailing a mess of female juices, cum and blood on the white sheets.

She probably should be angry, but she actually feels a mix of arousal and curiosity.

What happened here? Why?