Edward and Bella

I don't think I even have to do this one because we saw them get married she's literally their daughter-in-law, but:

Canonically Esme and Carlisle think of all of the Cullen kids as "their children" but they are also all married to each other so they can't literally all be their children or they would be married to their siblings.

my answer: bella is their daughter-in-law for now, but over the decades as she assimilates into the family and its ongoing cover stories, and as her own human parents die, she will grow into the role of "daughter." after all, it's not like they consider emmett their "son-in-law" just because he showed up later than rosalie. this is the vampire incest family