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 ╮(￣▽￣)╭
Ohhh, okay. So that's a possibility too. But i think she is their daughter cuz the duchess was pregnant if i remember correctly at the beginning of the story when sth happened to her. If she wasn't that's okay too, that way technically she is their 1st cousin lol.
The duchess died while giving birth to the twins
Judy said it in passing
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