I can believe that with people you barely know but not with friends.... you didn't know where they live? meet their parents? they never talked about family things? You never saw them together? Never went to their house? Maybe they weren't so close.
Today with Instagram and Facebook is even more difficult, you can't hide a Tybalt in real life for a second, the first thing any girl you meet does is... check your social networks to see who you are. In my country when people flirt they exchange the Instagram handle first and maybe later the phone number.
I mean it sounds stupid, but when you look at it it's nothing crazy, they were just different people with totally separate lives. The guy went to my class so we hung out at school and went out with other "school friends", mostly guys, and the girl went to the same school but was a year younger so we were never at school at the same time. I met her through other friends that had no connection to school at all, and just knew she was our fellow student, but never saw her in class or anything. We went clubbing a lot with that other friend group.
Outside of that they never really hung out together or mentioned each other, they just both had their own friend groups and had completely different interests. After I found out they were siblings, I got the feeling that they didn't dislike each other, just weren't close and didn't talk much at home, tho at that point he'd already moved out. Never visited their home while we were in school.
Not really a grand story or anything, just was a weird thing to connect when I first found out