You didn't. Your half smile looks like you didn't want your pic taken though and your sisters were smiling and more natural. You look similar to them though so why are you singling yourself out?
Maybe you think you don't look good in your picture because you are smiling. (I noticed in most your pictures you're not smiling). I do the same thing.. Only seem to smile in pictures when I'm caught off guard.
You look prettier than both of your sister...!!!