Yeah, Aeris did have some rebound issues BUT she did go after Cloud fair and square!
Aeris never saw (SPOILER)Zack again after he left for Nibelheim, five years before the events in the game. That's a heck of a long rebound period...
She was drawn to Cloud simply because he had those characteristics that she liked. Tifa knew him as a friend originally, and then wanted to stay by him and find out what was going on after he (SPOILER)turned up in Sector 7 carrying Zack's sword and with gaping holes in his memory. She became even more attached as a result.

So, I don't think either of them had a greater or more legitimate 'claim' to Cloud.

By the way, this may be too late, but please mark your spoilers.