Spoilers for chapter 1422 of Turning.
After all this time hinting in various ways that Yuder might be Helrem's deceased grandson, I don't believe Kuyu would give up on that idea, not when all that's left to do is confirm the kinship between the two.
I believe Kuyu added this other baby to the village where Yuder was rescued just to maintain the ambiguity that Yuder may or may not be Helrem's grandson, especially since the fact that Helrem never met her daughter-in-law or her grandson makes it even harder to definitively confirm their kinship.
And I think all this happens so that after the Nerlan arc we have the Pearl Tower arc, where they can both investigate whatever messes the mages are making there and try to find out more about Helrem's daughter-in-law, who was a mage trained at the Tower. Considering the possibility that Yuder has an identical appearance to his mother, there's a good chance that one of the Tower's mages, whether a former colleague or master, will look at Yuder, recognize his appearance, and exclaim, "You look just like so-and-so!", thus confirming that Yuder is indeed Helrem's grandson.