Here is another news item on Sarah, which I think leads to the discovery of another brother (of William, son of James Sr.)
https://www.newspapers.com/article/cambridge-chronicle-and-university-journ/161708965/At first I thought the Mary Ann in question was Mary, wife of James Jr., but it turns out that this Mary Ann was nee Dockrill, and married Thomas Taverner, who did die weeks before, as the newspaper stated.
All these Mary Ann's could explain why James Sr. put Sarah as a Mary Ann on the 1871 census, as Maddys noted earlier. His wife, a daughter, and so far 2 daughter-in-laws were all Mary Ann's. He was probably at the point of calling any female relative "Mary Ann"!
I haven't found solid proof that Thomas fits in this family (yet!).