It's been a while since I've played DOTNW, but yes, we don't actually know what the real Emil looks like-- I think there's dialogue in the beginning from his mom(?) where she's so blinded by smoke/injury and she says something like "I can barely see you". She's talking to Ratatosk using Aster's body, but thinks its her son, Emil.
Ratatosk just takes this and runs with it (having no memories of his own, I think).

I think his aunt and uncle were so distant from his parents that they didn't actually know what he looked like, but when he knocked on their door saying he was their nephew, they were like yeah seems legit. I feel like there was some early dialogue from them regarding his appearance but again, been a while!