Arthur Ransome Wiki
The term medicinal aunt is used for clarity on Arthur Ransome Wiki but is not the name of this character in the Swallows and Amazons series

The (un-named) medicinal Aunt of Jim Brading keeps a distant eye on her nephew, providing rum "for medicinal purposes only" to the crew of the Goblin in We Didn't Mean to Go to Sea. (This rum is used for a medicinal purpose to revive Sinbad and is later misused for a recreational purpose by Daddy and the Flushing pilot.)

At the beginning of Secret Water, Jim is said to be in the care of his aunt pending recovery from his head injury.

It is not clear if the two mentions of an aunt refer to the same person or two separate Aunts nor if either is related to or married to Uncle Bob. The Aunt(s) and Uncle seem to be Jim Brading's only living relatives.