Your sis goes into your room when you're not home and find a brand new bra. She is so happy but it's the wrong size. She calls you and you explain to her that it isn't for her, that would be weird. It is for you new girlfriend. She is mad and jealous? But why would your sis be jealous? You come home and she is in your bed with your girlfriends bra on. You tell her how weird she is being. One thing leads to another and the next thing you know you've been seduced by your sis and you're cumming inside her.