Jon Garth Murray (November 16, 1954 – September 29, 1995) was the second son of late controversial activist Madalyn Murray O'Hair, the first president and founder of American Atheists, Inc., in 1963. He was also the half-brother of the reverend William "Bill" Murray.
Madalyn Murry O'Hair is the mother of William "Bill" Murray
Although William is the half brother of Jon, there's no indication that Madalyn is William's mother.