How is it that my boyfriend's brother knew he was gay but my sister didn't know about me?

all of what youve named your boyfriend as being, deep voiced athletic confident etc etc, those are all features on the outside, your bfs brother, being his brother, knows him more than someone on the outside, know what i mean? his brother grew up with him, hes known him for him from a young age, not just a physically straight man, do you understand what im saying? your sister on the other hand is younger than you, and i dont mean she doesnt know you but i feel she did know you were different but she didnt know how to label it, or she couldnt put her finger on it. maybe she was trying to not ruin your moment. idk. everyone is different.