Aunt Tillie was the aunt of Otis and was only mentioned in the first game.
She gave Otis a carrot cake while he was imprisoned in the Mêlée Island jail. She managed to conceal a file inside it which was intended to be used to help him escape, but Otis left it aside due to hating carrot cake.
Guybrush Threepwood trades the cake with Otis for gopher repellant when he says he needs something to get rid of his rat problem in his cell. Guybrush finds the file within the carrot cake and uses it to obtain the "Idol of Many Hands".