Wouldn't she probably inherit both Poland and Hungary earlier?
Correct me if I'm wrong, but she was the daughter of Louis I of Hungary and Poland.
In OTL the realms were divided between his two daughters (Jadwiga and Mary). In this timeline, this ALT son would stand a chance of inheriting all his fathers domains.
EDIT: However, if I remember the books I read correctly, it was the Polish magnates that really pushed for the seperation between Poland and Hungary. They wanted a King IN Poland, not out of it like Louis. In this TL, they might opt for Mary instead, but that'd probably spark a war.
On the other hand, the Poles had never been fond of Sigismund, Mary's husband. Mary, like Jadwiga, died without issue. So even if the Poles opt for Mary and get out of it without a war (doubtfull), nature might just plop it back into this ALT son's hands again.