He loved me.......i refused...now 4 yrs later he's to get married to my sis....why do i feel bad about it?

hi...i'm a 24 yr old woman. we are three sisters and we grew up in a colorful neighborhood. we had this guy....a yr older to me...he was our childhood friend and went on to become my best friend. 4 yrs ago he confessed that he loved me. i refused because i had never thought of him as a suitor. he did try his best but then i held on to my guns. he's really handsome and rich and a really nice guy but somehow i wasn't ready for it. then he went away to study in another state and i too went away to another state to study. we guys did keep in touch and i tried hard not to change anything in our relationship. yesterday he and my youngest sister (she's 21) announced that they are engaged. i should be happy about it, i know...but i'm shocked....it hit me...i don't know why. i don't love him...i'm sure about it....then why am i not anywhere near elated? i love my sis and always want the best for her....but i feel so queer right now about the whole thing....why??