Mrs McDuck was a female anthropomorphic duck.
At some point, Scrooge McDuck's great-uncle Quagmire McDuck married this woman. Together, they had at least two children, the girls Elizabeth and Margaret, one of whom married a Mr McDivot while the other married a cousin of Brickheart Glomgold's.
Behind the scenes
Often posited by fan theorists, the existence of a wife of Quagmire McDuck was made official in the 2012 article Bogford McDivot.
It is often speculated that among Quagmire's other children were Pokerface McDuck and one or more other sons who became the fathers of some of Scrooge's more distant cousins, such as Moocher McDuck, Theobald McDuck and Melchior McDuck.