Christopher Ciccone spills the beans in his new book, Life With My Sister Madonna
Madonna's brother has said his sister would work hard to maintain her marriage to film director Guy Ritchie.
Christopher Ciccone, who has written a book about the pop star, made the comments on US TV network ABC.
"She's not the type of person just to walk away," he said, referring to reports that the couple's eight-year relationship was on the rocks.
Mr Ciccone also said he "doubted" reports Madonna was dating baseball player Alex Rodriguez.
Earlier this month, Madonna's spokeswoman denied speculation that the 49-year-old singer and Ritchie were preparing to divorce.
She also denied Madonna was dating New York Yankees player Rodriguez, 32, who recently parted from his wife.
Asked about the rumours, Mr Ciccone said: "I believe that she will do what's best for her family and her kids, and I think that she'll do her best to maintain the marriage and keep it going."
Mr Ciccone has been a dancer, designer and director for Madonna
He told the Good Morning America programme it was "highly unlikely" his sister was responsible for splitting up Rodriguez's marriage, adding the claims seemed "absurd".
"First of all, despite everything else, she's still a Catholic girl and I don't thinků although he is kind of her type, so it's difficult to say," Mr Ciccone said.
"I know Alex from Miami - we've met a number of times - I doubt this is actually an affair.
"I'm sure she had nothing to do with the break-up of their marriage."
Mr Ciccone also said Lock, Stock and Two Smoking Barrels director Ritchie had come between him and his sister.
Comments made during Ritchie's 2004 wedding to Madonna led Mr Ciccone to walk out of the reception, he added.
His book, Life With My Sister Madonna, is due to hit US shelves on Tuesday.
Mr Ciccone has also included gossip about Madonna's sex life and stories about various stars, including Sean Penn, Bruce Willis and Gwyneth Paltrow.
The singer, known for hits such as Like a Virgin and Holiday, first worked with her younger brother at the start of her career.
He also designed and directed her Girlie Show tour in 1993 and served as artistic director on her 1991 documentary, Madonna: Truth or Dare.
But in his book he said they were no longer close, and blamed Ritchie for breaking their close bond.