I like the sibset because it's so old fashioned, I could picture it for the daughters of a Revolutionary hero. I never quite understood
Polly as a NN for
Martha, but it's not a bad NN. If you want to keep that 1776 kind of feel, you could call her
Patsy, too.
Since this particular sibset is so old fashioned, you
could name
Sally Sarah to match Martha/Polly and Elizabeth/Betsy, but if you *really* don't like
Sarah, why bother? If
Susannah is more to your liking, that could work: Martha/Polly, Elizabeth/Betsy, and Susannah/Sally. Even though I think
Polly and
Sally are just fine on their own, it's a cute sibset. Go for it.