egalor said:
They're hardly a recent addition, nor something suddenly foisted onto the setting; the first mention of them (that I can find) is the Codex Imperialis that came with the 2nd edition 40k boxed set (released in 1992), and they didn't have so much as a codex or a piece of artwork until 5 years after that. It's not as if someone somewhere suddenly went "hey, we forgot to include women" and threw them fully-formed into the universe.
Besides, that argument can be applied to **** near anything in 40k - Eldar are Elves in space, Blood Angels play on the whole "vampires are [apparently] cool" shtick, and the whole thing is festooned with giant guns and all manner of other things to catch the attention and make people go "oooh..."
All that aside... if you scour the Adepta Sororitas (who, it should be remembered, aren't all warriors - the Orders Militant are only one part of the organisation) your version of the setting , what do you replace them with? Afterall, you can't easily have a "church militant" without an army of religious zealots...
Their hair styles are acceptable for our modern day military (a short bob) to the best of my knowledge. I think the uniform look was just a design element GW came up with to bring unity to the army. Also, the bad girls (sisters repentia and penitent engine pilots) look significantly less pretty than their sticture following sisters in the other combat units of the army so the entire force is not just an army of models... wait, they are an army of models, but I digress.