Balfour Paul thinks there was a 3rd son, name unknown. Nothing else known either.
https://archive.org/stream/scotspeeragefoun04pauluoft#page/162/
Apparently the evidence is only that William was called the 4th lawful son.
A golden opportunity served up on a plate :)
There seems to be a disagreement about the name of the sister, though her husband is agreed on.
Indeed there was
https://books.google.com/books?id=ujQOAwAAQBAJ&pg=PA17
with the usual assumption that everybody with the same surname is the same family.
We have Hugh
https://www.wikitree.com/wiki/Stewart-10472
but we don't give him a father.