The mother's are irrelevant. Women were automatically naturalized upon marriage. The fathers all became American citizens before their sons were born, therefore making them the son of an American citizen.
Not until after 1907. Prior to that an immigrant woman who married a US citizen did not automatically become a citizen, and there was little practical reason for them to become naturalized. They couldn't vote, couldn't hold property in their own names, etc., and naturalization was a fairly expensive process. So you cannot presume that they were citizens just by virtue of their marriages.