Roommate and i had sex,shes trying to get me to have sex w/ her sister

I am not going to use any names to keep this as anonymous as possible. I am a freshmen at college, and I am bisexual. I have been in a few relationships with girls through high school and have had sex with girls before. I live with one other girl and for the first semester everything was fine and we got along really well. We did make out a couple times at parties and we got really close really quickly. About a month ago I got back from class early and found her naked and masturbating to a picture of me. I didn't really freak out or anything and we talked for a while and she ended up eating me out. We have had sex basically everyday since then. Earlier this week I found out her sister (a senior in high school)is coming to visit and she just straight up asked me to have sex with her sister and told me she was going to film it. I laughed it off and really thought she was kidding and it didn't come up again until today when I came back to the dorm and saw a strap-on dildo on my desk with a note and a picture. the picture was of my roommate and her sister in bikinis and the note said "_______ is going to love you putting this in her" I think she is serious about this and actually I really really want to do it. Is this feeling normal,wanting to sleep with my girlfriend's sister? Should I do it? is it normal for her to want me to sleep with her sister?