The mystery surrounding the birth of Anakin Skywalker is one of the biggest mysteries in the Star Wars franchise. Anakin's miraculous birth and his unusually high concentration of Midichlorians make him the prophesied Chosen One that would one day bring balance to the force. However, the circumstances revolving around Anakin's birth are a lot more sinister than they initially appear with Emperor Palpatine playing a large role behind Anakin's conception, being similarly responsible for Rey's birth and making the two related through the force, based on this theory.
In The Phantom Menace, Qui-Gon encounters a vergence in the force centered around a young child on Tatooine, leading him to Anakin. Assuming this connection is tied through his genetics, Qui-Gon inquires about the identity of Anakin's father, but he gets an unexpected answer. According to Shmi, there is no father, and Anakin was inexplicably conceived without one. Qui-Gon believes the only explanation for this is that the midi-chlorians themselves willed Skywalker into existence.
Later in Revenge of the Sith, Chancellor Palpatine shares a story with Anakin about the tragedy of Darth Plagueis the Wise. Plagueis was a powerful Sith Lord who could influence the Midichlorians in the force to create life, and such knowledge meant he could save people from dying. He grew so powerful that he became paranoid and believed the force would strike back at him, but it was his apprentice who killed him in his sleep before passing on his knowledge.
Qui-Gon's theory relating to Anakin's conception is turned on its head when taking Darth Plagueis' story into account. Plagueis was apparently able to create life through his manipulation of the Midichlorians, and Anakin is also believed to have been created through this method, which Palpatine's story alludes to. Palpatine's story seems to imply that he could be Anakin's "father," being somewhat responsible for the circumstance that led to Anakin's creation.
Palpatine was Darth Plagueis's apprentice, and they spent years searching for the secrets behind immortality. During their efforts, Plagueis discovered the secret to eternal life by transferring one's consciousness to another body and using it as a vessel. In the Darth Plageuis novel written by James Luceno, Plagueis and Sidious attempted to will a being into creation by pouring their malicious intent and dark energy across the galaxy. Their attempt backfired and caused the Midichlorians to create a Chosen One that would destroy the Sith and bring balance back to the force.
The Darth Plagueis novel was originally supposed to be released in 2008, but it ultimately came out in 2012. Rumors began circulating that Luceno worked closely with George Lucas when working on the novel, but when Disney bought Lucasfilm, all of the Expanded Universe material was declared non-canon, so the validity of the book was put into question.
However, the events of Darth Plagueis are explicitly referenced in the canon Tarkin book, and in Darth Vader #25, Vader enters an ethereal realm where he relives pivotal moments of his life and sees an image of Darth Sidious encircling a very pregnant Shmi Skywalker. This comic seemingly confirms Palpatine was Anakin's "father;" however, Story Group executive Matt Martin claims the implication was never meant to be taken literally.
Similar to Anakin, Rey's parentage is quite complicated to discern. Palpatine cheats death on the Death Star by transferring his consciousness into a clone body, and his initial soul transfer leaves him in a decrepit body as his immense power is too much to be contained. The Sith Eternals try crafting other clones for the Sith Lord, but the results are monstrous; however, they manage to create one non-identical clone, but he lacks force sensitivity. This "son" would eventually meet a woman and have a force-sensitive daughter named Rey.
Anakin and Rey have similar origins as Palpatine looms over much of their lives and is responsible for their creation. In a sense, these two are force siblings despite not sharing the same blood. The entire Skywalker family is finally able to defeat Palpatine through Rey's actions, and the connection these two families share are intertwined, making Rey in a way Luke's aunt, based on this theory.