From PEOPLE Magazine Click to enlarge
Lettie Martinez takes a deep breath and blows out the candles on her chocolate cake. Serenaded with "Happy Birthday" by her large extended family, she claps her hands and beams. "How old are you, Lettie?" a cousin calls out. Lettie ponders a moment, then replies, "Five! Six!" Lettie's sister Susana smiles and says, "Lettie's been talking about this birthday for months. She's 56, but she's really just a happy 5-year-old."

Born with cerebral palsy that resulted in intellectual disabilities, Lettie lives with her sister Susana, 54, whom the rest of the state of New Mexico knows as Governor Martinez. The country's first Hispanic female governor, Martinez is a rising star in the Republican party who enjoyed national exposure at the 2012 Convention. "She can be seen as the future of the GOP," says Republican political strategist Ford O'Connell. "She has the potential to be a candidate who can bring the whole party together."

As she relaxes away from the Governor's mansion, shoes off, in her Las Cruces home, Martinez says she has heard the political chatter and is humbled by it. Still, she is firm in stating what is paramount in her life. "It's all about my family," she says. "Especially Lettie."

Lettie moved in with Susana and husband Chuck Franco, 57, a retired law-enforcement officer, when the sisters' mother died seven years ago. (The three frequently shuttle to the capitol in Santa Fe.) But Susana's caregiver role has lasted a lifetime. "My parents had their own security business and worked hard to build it," she says. "So I always was in charge of her." As a teen that meant bathing Lettie, brushing her teeth, helping her dress. (They now have a companion to help with the daily routine.) When Lettie went to summer camp for children with special needs, Susana volunteered as a counselor. But she doesn't dwell on the burdens so much as the joys, such as cheering her sister, then 16, in the Special Olympics. "Basketball, long jump, Frisbee. She could shoot, and I was like, 'Where did you learn to shoot a basketball?'"

On this June day, she is in charge of making her sister's birthday special. The two go shopping, to lunch and to a spa. Susana remembers Lettie's being uncomfortable in public when they were kids. "Back then people stared a lot, and she used to hate it: 'Why are they staring at me?' Now people are a lot more informed." Most are also more interested in Lettie's sister, who stops frequently to talk with constituents. When she does, Lettie steps away to pet a shelter kitten or eat ice cream while showing off her newly painted toenails. "I love red!" she says.

Susana, Leticia and brother Jacob, 58, and the owner of a uniform store, grew up in a working class neighborhood in El Paso, the children of Paula and Jacobo Martinez, who died last year after a lengthy battle with Alzheimer's. "My dad had been a champion Golden Gloves boxer, then a deputy with the Sheriff's Department," says Susana. "My mom worked for an insurance company." After they started their security business, Susana, then in high school, "took a course to qualify to carry a firearm and was one of my dad's first guards. I would walk the parking lot at a big Catholic church bingo and make sure no one was breaking into cars."

By the ninth grade, Martinez says she had a clear career goal: U.S. Senator. After graduating from the University of Oklahoma College of Law in 1986 (and a brief first marriage), Martinez joined the district attorney's office in Las Cruces, where she made a name for herself as an aggressive prosecutor of child-abuse cases. There she met Franco, at the time an undercover officer with the sheriff's department. "The first time I saw Chuck, he was dressed as a gang-banger—muscle shirt, tattoos, hair net—it was not love at first sight," she says. But they wed in 1991, making the decision not to have children. Chuck had a son from a prior marriage, and Susana says, "I knew that after my mother died, Lettie would come back to me, and she would still be 5."

Until she ran for her first office - elected District Attorney in 1996 - Martinez had been a Democrat. She surprised her family by switching parties after a lunch with two local GOP leaders. "After it was over, Chuck and I got in the car, and I said, 'I'll be damned. We're Republican. This is not good.'" She worried about winning over the county's Democratic majority but did - and held her post for four terms.

Martinez won her state's top job in 2009 and in 2012 she was mentioned as a possible Mitt Romney running mate. Should her name come up in 2016, Martinez says she'll be flattered, but "Lettie will always be my priority." Today that means hanging out in the school-supply section of Walgreens. "Even though she doesn't write, Lettie loves spiral notebooks. I don't know what the intrigue is, but I buy them. But her favorite thing is sitting in bed watching movies with me, as we've done for years and years and years. If Chuck is out of town, she sleeps in the bed with me. It's just what we do."