I think that they technically could be Webby's daughters if Webby was older. I think it mainly boils down to age, and whatever title they feel most comfortable with.
Either way, it's implied that Donald adopted June and May, making them legally Webby's first-cousins-once-removed, even if biologically they're something else. I can see Webby just calling them that bc honestly I feel like they'd all be confused about what they are to each other too.
