Millennial Relationship Issues

So short and simply folks i met this girl we dated for a while. Eventually i began to hang around her family, and i met her sisters. One night , they revealed to me that they had a threesome w her sisters boyfriend, and i was disgusted, but I figured im not an unlucky guy anyways and i shouldn’t be so insecure about something from her past, and atleast they had the decency to tell me.

Fast forward, her sister and her move in with me, and when her sister suggests that she wants to have a threesome , she gets angry and threatens to cut both of us off and to never ask again…

am i overreacting for feeing like she doesnt value me the same as the other guy?? I feel like the other guy must’ve had something i dont or something idk its hard to tell. Her sister is obviously attracted to me and had hit on me and tried to have xes w me multiple times but i wanted to be honest about it like they were with me so i didnt touch her until we asked my girl . Now i feel like i shouldve just done it.

(fyi her sister has seen me in my birthday suit plenty of times so i doubt it has anything to do with my body or anything like that. I dont know what to feel besides “she was willing to let the other guy do it but not me” Neither me nor her sister understand her decision….)

How should i move forward (IF i move forward) with the relationship? How should i feel? am i not looking through the correct lens at all this deviancy or am i simply overreacting ??