lean on your writing rather that appearance to make them different. and after you manage to make them internally different it's a breeze to emphasize that difference in appearance. the tough girl is gonna look very different from quiet nice girl, even if they were twins.And yeah, I wanted to have them be related but add a little variety to it. My challenge is to have variety in the umpteen women who are gonna be core or side characters. The hardest thing is body diversity for me. I have a hard time making their breasts smaller without screwing up the aesthetics of them. I have some 'go-to' body morphs but they can get a little samey and difficult to change details effectively.
if you make the exterior first you're kinda stuck with some 'ideal woman' in your head, and everyone will tend to gravitate towards that one look because there's nothing internal that's guiding your design elsewhere.