Well you (By 'you' I mean the PC) don't even know they were related until you meet Anno anyways, and you don't really know they are twins until Syri tells ya. That's kinda the point, you don't know until they actually tell you.
Also, have ya ever heard of Fraternal Twins? Two people that ARE twins but look nothing alike to be classed as identical? That's actually a thing that exists, and that's what the Dornas are. Identical twins don't exactly exist in dog litters like real dogs, which is what Ausar tend to imitate because doge people and they have litters. Near every pup in a regular dog's litter is fraternal, no one pup in a litter is exactly identical in any way, shape, or form..
Not all twins are identical twins, ya know. I'm not entirely sure why there are those that think one isn't a twin unless they look alike. It's all down to when they were born. Whether they look entirely identical or not, if they were born on the same day, they're twins regardless of how different they are to the eye.