Balfour Paul thinks there was a 3rd son, name unknown. Nothing else known either.
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
with the usual assumption that everybody with the same surname is the same family.
We have Hugh
but we don't give him a father.