Kanto's most terrifying Gym Leader might've been throwing her brother into battle, if this classic Pokémon fan theory is to be believed. The idea that Sabrina's Kadabra is in fact her brother was first posted onto the Pokémon Conspiracies subreddit years ago by user MarvelSyrin. Sabrina and her Kadabra -- who first appears as an Abra -- are shown to have an intense psychic bond, to the point where the two share emotional states. When Haunter makes Sabrina laugh, Kadabra also laughs.
The theory argues this closeness might be due to how Sabrina's Pokémon is also her brother. This might sound absolutely absurd, but there is possible evidence in the Pokédex indicating that her Kadabra could be a former human. However, is that enough evidence to make this almost ten-year-old theory sound convincing?
A Pokédex Entry Shows Some Kadabra Might Be Ex-Humans
Throughout multiple entries in the Pokédex, it is indicated that people have transformed into Kadabra before. The first mentions of this, however, are in Gen 3's games. Pokémon Emerald's Pokédex entry reads "It is rumored that a boy with psychic abilities suddenly transformed into Kadabra while he was assisting research into extrasensory powers." More famously, however, in Pokémon FireRed, Kadabra's Pokédex entry says "It happened one morning - a boy with extrasensory powers awoke in bed transformed into Kadabra."
Later games only further reinforce this rumor, often adding new elements to the Kadabra myth. In Pokémon Sun, however, we come to the closest relevant entry that might explain how Sabrina's theoretical brother could have transformed into Kadabra. The Pokémon Sun Pokédex says "A theory exists that this Pokémon was a young boy who couldn't control his psychic powers and ended up transformed into this Pokémon."
Psychic power appears to run throughout Sabrina's family. As a result, it seems very likely that her brother could have lost control of his psychic abilities and transformed. That would also explain both Sabrina's obsession with honing her power and her estrangement from her family. She didn't want to transform and could resent her family for her brother's fate.
Of course, all of this theorizing ignores the obvious flaw with this theory: Sabrina's Kadabra starts as an Abra. In all the games' Pokédex entries, there is no indication that any child has transformed into an Abra. Nor, for that matter, is there any sign that any Kadabra actually was a human first before transforming, outside of urban legends perpetuated by the Pokédex.
Sabrina and her Kadabra Are Close For A More Obvious Reason
As of yet, there is no evidence Sabrina even had a brother. He doesn't appear in any of the photos shown of Sabrina's family. While we meet both of Sabrina's parents, we never hear any mention of a brother. If there's no brother, then why is Sabrina so close to her Kadabra? The answer might be pretty obvious, and, once again, have a basis in the Pokédex.
Sabrina is an incredible psychic. Experts as well as those with latent psychic abilities can often communicate with Psychic Pokémon far more effectively in Pokémon's canon. This is, once again, further reinforced by reading the Pokédex. Pokémon Ruby's Pokédex entry reads "Kadabra emits a peculiar alpha wave if it develops a headache. Only those people with a particularly strong psyche can hope to become a Trainer of this Pokémon."
The reason Sabrina and her Kadabra share a psychic link is because the two are incredible Psychics able to link up together. This is, again, the canon explanation in the anime. While the brother theory sounds fun, there is just no basis for it to be real in the canon. That said, if Sabrina had a brother, there's no saying he wouldn't meet the same fate as the rest of Sabrina's family: estranged or preserved as a doll.