Evelyn McDuck was a female anthropomorphic duck.
Believed to be the eldest of her family (and thus to already have left home by the time Scrooge turned ten), Evelyn was presumably Scrooge McDuck's sister and therefore the maternal aunt of Donald Duck. Wealthy and very fond of her cat, she passed away in 1973, leaving instructions in her will that her beloved cat Jorge was to be the heir to most of her fortune, while she left mere sentimental keepsakes to her family (such as her childhood doll to Donald). However, any family member who agreed to take in Jorge and take care of him would receive a $1000 monthly pension.
Behind the scenes
Evelyn McDuck was only ever mentioned in the 1973 story Feline Felony.
Though the story makes no pronouncements about Evelyn's age, it stands to reason she would be the oldest of the McDucks, having already left the family home by 1877; this would explain her absence in depictions of Scrooge's youth such as The Last of the Clan McDuck.
As far as can be parsed from the Portuguese version of Feline Felony (at present the only one available to editors of this Wiki), where Evelyn is called "Aunt Patonsina", there is no mention of Evelyn being a maternal aunt.
Some speculate that Evelyn may simply be an alternative name for Matilda McDuck, which would mean Matilda is deceased in the present-day.