I don't see this becoming an incest thing. KissxSis they used the "not by blood" trope.
But this, despite being a completely different girl/woman, is still his birth mother. It'll likely be something along the lines of "staying in till I know my son is doing fine."
You'll get the romance with the obvious love-interest and the sad parting where the mother says she has no regrets and departs for the afterlife. But, plot twist, she never actually left.
This will all end with some comedic shenanigan like a tween asking when her newly married son will give her grandkids.