According to the Wikipedia article, here,
https://en.wikipedia.org/wiki/Mormonism_and_Freemasonry, yes their father was a Freemason. "The father, Joseph Smith, Sr., was a documented member in upstate New York. He was raised to the degree of Master Mason on May 7, 1818 in Ontario Lodge No. 23 of Canandaigua, New York."
But claiming that Hyrum wrote the Book of Mormon doesn't explain anything in my opinion. It maintains all of the same questions about how an ignorant farmer could produce such a document. And it adds new questions--why would Hyrum do this? Why would he continue to support his brother (younger brother, even) Joseph as a prophet for his whole life, to such an extent that he would even die with him? Plus, I don't even think Hyrum was around the whole time as the Book of Mormon was being written down on paper.