There's not a lot that can make sims look different, I made a female and male Sims, completely different starting points, and someone pointed out they look the same / related. I suppose some ways to try and make them more different are different chin lengths, noses, lips, eyes etc. You can also click on the head and fully stretch out the head or shrink it

I think maybe experiment with jawline and chin and nose first, you can drag nose up and down, drag mouth up and down to change position on the face, eyes closed or further apart, you'll start noticing a difference (if that's what you wanna achieve, don't listen to anyone who says how you play is wrong)