Why not pegasister?

As a portmanteau of pegasus and sister, the word pegasister, when used to describe female bronies as a whole, implies that all female bronies are pegasi. Unicorns and Earth ponies are being excluded.

Why ponysister?

It sounds strikingly like pegasister, but it includes every female brony, regardless of pony species. It’s simple, clear, and easy to spell.