The 1880 US Federal Census seems to confirm that Clara E. Wright was his widowed mother. She was born in Indiana abt 1848, according to the Census. It indicates Sherman's father was also born in Indiana.
Source Citation: Year: 1880; Census Place: Northeast, Orange, Indiana; Roll: 302; Family History Film: 1254302; Page: 65A; Enumeration District: 013; Image: 0397.