Madonna's brother Christopher Ciccone says the singer would "do her best to maintain" her marriage to British director Guy Ritchie and that he doubts she had an affair with Yankee baseball player Alex Rodriguez.

Madonna and Ritchie have been plagued by persistent tabloid rumors that their marriage is to end, prompting the singer to release a statement saying she is not planning to divorce her husband and denying an affair with Rodriguez.

Ciccone, who says he has had little to do with his sister since she married Ritchie in December 2000, has written a memoir called "Life with my sister Madonna," which will be published by Simon Spotlight Entertainment on Tuesday.

"I believe she'll do what's best for her family and her kids and I believe she'll do her best to maintain the marriage," Ciccone told ABC's "Good Morning America" on Monday. "She's not the kind of person to walk away."

Rumors that the London-based couple planned to split have been circulating for months, fueled most recently by reports that the singer had lined up Fiona Shackleton, a lawyer for Paul McCartney in his bitter divorce from Heather Mills.

Then after reports that Rodriguez, the Yankee's power-hitting third baseman, had been spending time with Madonna, his wife filed for divorce.

"I doubt this is actually an affair," Ciccone said. "I'm sure she had nothing to do with the breakup of their marriage."

Madonna, 49, and Ritchie, 39, married in December 2000 at Skibo Castle in northeast Scotland. They met at a party hosted by ex-Police frontman Sting and his wife Trudie Styler.

Their son, Rocco, was several months old on their wedding day, and in 2006 Madonna applied to adopt David Banda, a boy from Malawi whose mother died shortly after his birth. The adoption was approved by a Malawian court in May.

Madonna also has a daughter, Lourdes, from an earlier relationship. She was married once before, to Hollywood actor Sean Penn, in the 1980s.

Madonna is one of the most successful rock stars of all time, with global album sales estimated at more than 200 million copies. A multi-Grammy award winner, she was inducted into the Rock and Roll Hall of Fame in March.

Ritchie is best known for his movies "Lock, Stock and Two Smoking Barrels" (1998) and "Snatch" (2000), but he has come in for critical maulings since then, most notably when he directed Madonna in "Swept Away" (2002).

Rodriguez signed a 10-year, $275 million contract with the Yankees in December making him the highest-paid player in baseball.

Source: Reuters/Nielsen