Who is Miss Nibbly and Frank Nibbly from The Mostly True Adventures of Homer P. Figg and what is their importance?

Miss Nibbly is a beautiful con artist who targets Mr. Willow on the dock before he and Homer board the steamship. Miss Nibbly learns why Mr. Willow is traveling with Homer and that Mr. Willow has a large amount of money to buy Harold back, if that should be necessary. Miss Nibbly learns about this money and she quickly convinces Mr. Willow to marry her and give the money to her brother, Frank Nibbly. Mr. Willow does, but later learns the Nibblys are not who they said they were.