Brilliant Earth scientist, and sister of Rodney McKay. Though a civilian and mother, with no formal ties to U.S. government agencies or the military, Jeannie is now a helpful, long-distance contributor to McKay's work in the Pegasus Galaxy. She married Kaleb Miller at a young age, rather than joining her brother in higher academia, and has a young daughter named Madison.
Jeannie's relationship with her older brother was estranged for many years, with Rodney admitting early on in the Atlantis expedition that he and his sister were not close. In spite of this, the only piece of video he wanted to be sure made it home to Earth if Atlantis was destroyed in the Wraith siege was a message to her. In the clip, he emphasized how he hoped he could get to know her better, and perhaps be closer to her, after he had returned from his duties.
The two were finally reunited nearly two years later, when a brilliant math proof that Jeannie did (with her daughter's finger paints) brought Rodney back to Earth and to her doorstep. She was reluctant to let him back into her life, since he had always harshly judged her choices in life. But after he transported her to the Daedalus and introduced her to the Stargate program, the two had to bury the hatchet and work together. With Jeannie's help, they pioneered an interdimensional energy collector, designed to generate enormous amounts of power by harvesting vacuum energy from a parallel space-time.
Through their work together their relationship was reconciled, with her brother -- whose real name she revealed to be Meredith -- repenting for how he had treated her. Jeannie then returned home to her family.
A year or so later an e-mail from Rodney, asking for assistance with nanite programming, put Jeannie in danger. She was kidnapped by Henry Wallace, head of Devlin Medical Technologies, who's daughter Sharon was dying. She had been injected with prototype nanites to try and cure her leukemia but they weren't working. Jeannie had been kidnapped so she could fix the coding. Rodney was also taken prisoner when he traveled to Earth to help in the search for his sister. After a failed escape attempt the two were eventually rescued.
Jeannie continues to maintain a relationship with Stargate Command and the Atlantis expedition, should need for her skills arise.
Hot Zone - Believing he is about to die, Rodney tells his coworkers that someone should tell his sister back home what happened to him -- and to make it sound heroic.
Letters From Pegasus - Rodney records a personal note to his sister, Jeannie.
McKay and Mrs. Miller - When Jeannie is recruited by the Air Force, Rodney must reconcile with her and convince her to visit Atlantis to help him on a project.
Miller's Crossing - Jeannie is abducted from her home in the night and held against her will by a wealthy industrialist, who demands that he and her brother use experimental nanite technology to save his daughter's life.