So, there was a group of guys (more than two). One of them smiled at your sister, but not at you. He thought your sister was cute, or she reminded her of someone he knows, or whatever.
But what does that say about the other guys? They were not rejecting your sister-or you-for not smiling at either of you. There are as many reasons why someone might smile (or not smile) at a total stranger as there are people, and none of it has to do with your value as a person.
So what should be your take away from this (trivial) incident? I can’t speak for you, but when this sort of thing has happened to me, I hardly noticed, because my self worth does not depend on the random actions of strangers.
I agree with @ScienceChick. In the grand scheme of things, this is only a big deal if you make it a big deal. Perhaps you need to find someone you can trust and get help.