Bestfriend's fiance tried to get with me!

So my best friend and her sister have an apartment together. Her sister is engaged to a guy she's been with almost 7 years, and they plan to move in together after the wedding. I met her fiance about 5 years ago at a gathering my bf's family was having so he is familiar with me. Anyway, the flirting started when I would go over to visit my bf at their apartment. He would occasionally be there and started bumping into me whenever we were passing each other in the hallway or fixing plates in the kitchen, etc. He would also watch me and my every move every time I would get up from the sofa..but I didn't think anything of it at first because my bf and her family are like family to me. They all call me sister and I'm always at their family events,etc. I just assumed he bumped me on accident or was just looking. So one day, he sent me a request on FB, I accepted, then he messaged me saying how beautiful I was, how he wanted to take me to dinner. So I'm like oh shit, I know this fool knows who I am.. So I told my bf to see what she would say, and she ended up telling her sister (his fiance) he claimed he didn't know it was me because my pictures look different and I don't go by my name on FB. Really? Like I look the exact same and even if he couldn't tell...obviously he went through all of my pics and saw pics of me and my bf and his fiance. And this isn't the first time he's been caught. Previously, my bfs sister caught him having sex, in bed, with her (now ex bestfriend). She went to his house one day unexpectedly and caught them together, butt naked. So my bf sister acts like it never happened and still speaks to me like he never tried me but still has him over sometimes. Idk, I just feel awkward going over there now especially when he's there now. Even though no words are said, I still feel tension when he's over. Idk whether my bf's sister doesn't care about what he does or what she thinks about it all b/c she never says anything so its kinda scary. To me, that would all be signs not to marry him but whatever floats her boat I guess. I just hate I can't really go over there like I used to without feeling uncomfortable. (My bf doesn't have a car so I would always just drive to her place which is a 20 min drive from me and chill...so now we don't see each other as much).