Illegal Guardian

Our favorite Heartwarming Orphan has recently lost his beloved parents and is in danger of being sent to an Orphanage of Fear. But what's this? That uncle we've never heard of has agreed to be our legal guardian! The family fortune is saved! We just have to wait until we're 18 and... what's uncle doing with that axe?

This is the Illegal Guardian, who may be an Evil Uncle or Wicked Stepmother or no relative at all with an Evil Plan to get all the money from those darn cough, cough beloved children.

Examples of Illegal Guardian include:

Anime and Manga

Hana is adopted by a priest after her mother is arrested. The entire family is abusive, and the priest's main motivation for keeping Hana around is for the welfare payments.
Subverted by Magical Girl Lyrical Nanoha. Hayate's guardian is Admiral Gil Graham of the TSAB, who set her up in a nice house, paid all her bills for her, and generally gave her the comfiest life a crippled preteen could ask for. Then he put her through a Heroic BSOD and Demonic Possession and nearly had her sealed away between the dimensions for the rest of eternity. The subversion comes about because Graham genuinely wanted Hayate to have a good life before he did this, and he viewed his actions as being regrettable but necessary in order to save billions of lives from the Artifact of Doom possessing Hayate.

The abusive foster parents in the Sandman volume The Doll's House.
Beast Boy's legal guardian; from the original Doom Patrol series and referenced in Teen Titans.

Count Olaf
Sheridan LeFanu's Uncle Silas. Played with a bit, as the dying father intentionally leaves his daughter in his brother's care to clear his name, as he thinks Silas was wrongfully accused of murder years ago.
Roald Dahl's Matilda features the terrifying Agatha Trunchbull, evil aunt to the angelic Miss Honey, who fits this trope to a tee. Not only did she force Miss Honey to do all the housework as a child, she also mistreated her, leaving her a nervous wreck. All this after she had killed her brother-in-law in order to get her hands on Magnus' house and money by forcing the emotionally battered Miss Honey to forfeit all rights to her inheritance.
The main character of The Dresden Files spent the last four years of his childhood living with one of these, though Justin DuMorne was after magically powerful slaves rather than money.
In Harry Potter and the Philosopher's Stone, Harry believed his Uncle and his Aunt would have stolen his father's fortune if they knew about it. Given how they treated him, it's a reasonable fear.
In Animorphs Tobias had previously lived with an aunt or uncle, neither of whom cared much about him. Then a long-lost cousin suddenly appears who seems interested in taking him in...only to be revealed to be Visser Three in morph, who became interested in Tobias because he's Elfangor's son.

Richard III, making another one for The Zeroth Law of Trope Examples
- Claudius, in Hamlet. He wouldn't need to abuse his position as Hamlet's stepfather, since he's also the king, but he finds it politically advantageous to do so rather than ordering the technical-rightful-king around.
Mime from Richard Wagner's Siegfried. A Dirty Coward who tends to be (wrongly) made into The Woobie in modern productions.
Rooster Hannigan and Lily St. Regis, from Annie, pretend to be Little Orphan Annie's "real parents" to scam reward money out of Daddy Warbucks.
Uncle Barnaby in Babes in Toyland (though not the 1934 or 1961 film adaptations).

The butler in The Aristocats
Sylvester Sneekly from The Perils of Penelope Pitstop
A movie for The Littles focused on Tom and Lucy befriending an orphan boy who's abused and locked up by his uncle, who only took him in for the access to boy's inheritance. Later the boy finds a letter from his father saying that his uncle was never meant to adopt him, someone else was.
Tom and Jerry helped a girl who lived with that kind of guardian in The Movie.