What's crazy about the Sasha-Gemma resemblance is that when they are separated, they definitely seem like two incredibly different people. Their eyes, cheekbones, noses, face shapes noticeably contrast.
Sasha's face is very angular, with high cheekbones, catlike eyes, a straight and thin nose, etc. She has a more mature or feline look.
Whilst Gemma's face is a lot softer, with round cheeks, doe eyes, a short button nose, etc. She has a younger or more doll-like look.
The differences are clear, but when they're together (and even more when they have similar makeup) it's a whole different story:
You have to look close to tell them apart, it's crazy! The obvious differences haven't disappeared and are actually very visible, but still...