I think a lot of your chances may be impacted upon which school you are attending. I don't know if being "older" than the traditional freshman would effect your chances at a school like UF or FSU, as compared to a much less competitive greek system like what we have here at Florida Tech, where it wouldn't be a problem.
I think, though, that if the sisters see that you are a freshman and would be able to be active for the full 4 years, being an "older" freshman might not matter...
I imagine that you don't look much older than an 18 yo. So unless the question comes up, age doesn't matter. You are a freshman, and therefore would have 4 years to give.
As a freshman, most would assume you just graduated, and therefore wouldn't bother to ask anyway.
Just don't volunteer the info. If they ask (which I doubt they will), be truthful. I wouldn't think it too weird if your big sister was younger. Bigs are there more for you to have someone to guide you through pledging, not to give you worldly advice. And chances are, you may have a Big sister who is your age.