Ruth Carter Stapleton turned to her brother during the Inauguration and convinced him to give her exclusive rights to write his biography.

Not her brother the President but her brother the gas station owner. Before the rest of the United States discovered the wit and wisdom of Billy Carter, Stapleton was struck by the different paths her two brothers had taken in life. And their public images notwithstanding, both Billy and Jimmy are of equal intelligence, their sister vows. So on Inauguration eve she got Billy to sign an agreement permitting her to write his biography. Then she spent a week walking with him, taping their conversations in Georgia.

"Mostly I just laughed the whole time," Stapleton says. "There wasn't all that much on the tapes I could use."

In the rough draft of her book, which Harper & Row plans to publish next year, Stapleton writes of the Billy no one knows, a quiet man "who has a whole day for a person who has had a need, the Billy that is everybody's friend." Her working titles: My Brother Billy, or The Billy I Know or just plain Billy .

Stapleton has also been bargaining quietly to purchase a ranch outside of Dallas as a center "ministering to the whole person, with an emphasis on inner healing and physical balance."

TTentatively called the Holavista Ranch, the center will accommodate about fifty people at a time. Stapleton will teach one session a month.

Since her ministry gained attention following Jimmy Carter's rise to prominence, Stapleton has maintained a hectic international schedule of lectures and personal appearances. "I'm the first one," she says wearily, "who is going to retreat to that ranch."