Well, Scrooge is their mother's uncle (their granduncle), and if Webby is his genetically cloned offspring, then, technically she's their genetically cloned/modified aunt once removed and Della and Donald's genetically cloned/modified first cousin once removed. Maybe this will help.

"Family," same as they were before. I don't think the exact genealogical relation is all that significant as it stands. But if you want to get technical, if Webby is to be seen as Scrooge's daughter, than she is first cousin to Donald, Della, Gladstone and Fethry, which makes her first cousins once removed to the triplets.

Of course, even Scrooge and Webby's mutual relation is technically not that simple. Webby was cloned from Scrooge, but Webby sees Scrooge as her "dad." And yet May and June were cloned from Webby, and yet Webby is not their "mother," but their "sister." What's the difference? Scrooge is so much older than Webby, but May and June are of a similar physical and mental maturity level as Webby and Webby herself is still a fairly young child, so it makes a sort of contextual social sense for one cloning to be treated like a father-daughter relation and for the other cloning to be treated like a sibling relation. Still, on a technical level, this is entirely arbitrary. Clones are neither strictly offspring nor siblings, but more simply copies. Even if those copies may have a few modifications (such as a difference in gender), they're still effectively copies.

Compare the case of Magica and Lena. Lena is a magical shadow clone of Magica. But is Magica Lena's "mother?" No. Is she Lena's "sister?" No. She's Lena's "aunt." While an aunt and niece are family, they are not necessarily expected to be part of the same immediate nuclear family, so while an aunt might be considered to have more authority than a niece in a larger family structure, she would not usually be expected to have the same caring familial responsibilities as a mother or sister. I think this was entirely for Magica's own convenience, making it easier to compel at least some of Lena's loyalty while being able to justify a lesser display of concern for Lena's upbringing and welfare. Indeed, I think the truest thing Magica said was that she did not consider Lena to be her family at all, right before trying to destroy Lena's shadow body. Later still, after Magica had been stripped of her powers, but Lena's body had been restored with magic powers of her own, and Magica wanted to claim Lena's powers for her own, suddenly Magica again declared that Lena was "family." That is a kind of relationship, but I wouldn't call it "mother," "sister" or even "aunt." It's something more like "someone please call a social worker."

I think she's technically their first cousin once removed

I've given this some additional thought since my last comment.

From a standpoint of genetic diversity, clones are actually more like siblings than offspring, as they only inherit from one parent rather than from more than one; her genes are still about half from Fergus McDuck and half from Downy McDuck, the same as for Scrooge himself and his own siblings. So, if Webby's genes do not actually differ all that much from Scrooge's, they could be genetically screened for medical purposes in the same way as genetic siblings, or perhaps even as identical twins barring the few genetic differences that do exist.

But of course, that's just the numbers of it all. Given their great differences in age and maturity, and that Scrooge was the template for Webby, and that Webby has lived most of her life as a dependent under Scrooge's roof, it seems more appropriate to liken Scrooge as her father.

It stands to reason, if Webby is his "daughter", then they would have needed an additional DNA source, otherwise they would have gotten an exact clone, but Webby, April, and June are c;early not cloned Scrooge's. And Scrooge did say "heir", and a clone isn't an heir (not in the standard sense, as heir typically refers to either offspring or the one to inherit). So there had to have been someone else's DNA mixed in. Maybe Goldie's?

@Ice-jin If Goldie was Webby's mom I would be so happy for some reason. It would make the beginning half of "The Forgotten Fountain of the Foreverglades" sooooo much better

Well, Scrooge did specify "Rightful heir". And seeing as Goldie is really Scrooge';s only love interest (and seemingly most compatible with him based on their personalities, traits, love for adventure, etc), she would likely be the most logical choice in creating the "rightful heir".