Responses
This is just my opinion/conclusion.. Maybe the other women on the portrait was the twin sister/sibling of the duchess.. then that person was missing or they thought she's dead.. But the truth is she was the mother of Esther, so technically, they're relatives..
I just saw some spoilers down the comment thread ╮(￣▽￣)╭
She looks so much like the duchess, or the woman beside her (idk who she is) why they didn't even hesitate to assume she could be their lost daughter? That's a little disappointing, but it will be fun when they'll realise later on. Can't wait for that