Many years ago,
gabrielleabelle made a poll about details of Spike and pre-Spike William's life. One of the questions was whether or not William had siblings - and if so, what gender. It turns out I have an opinion on this, which I might not have realized had I not been asked the question.
I think William had an older brother who died before he was born. One of those childhood illnesses that took so many lives in the days before vaccinations. The loss made his mother more protective, but William never got the chance to know his brother.