(by Sarah Muller)

It’s one thing to steal your sister’s diary to see all her dirty secrets. It’s another to write a tell-all book filled with dirty secrets for the entire world to read.

Madonna’s baby brother, Christopher Ciccone, is now writing a juicy memoir about his famous sibling. An imprint of Simon & Schuster plans to release 350,000 copies of “Life With My Sister Madonna” in mid-July. Ciccone has been working alongside Wendy Leigh, who has written celeb bios on Liza Minnelli, Grace Kelly and Arnold Schwarzenegger.

No word on how much money he got for ratting out his sister.

Madonna and her bro used to be pretty tight. Ciccone designed and directed her “Girlie Show” Tour in 1993 and served as artistic director on the “Madonna: Truth or Dare” doc in 1991.

Madonna’s spokeswoman Liz Rosenberg has said they are no longer close. Yeah, not after this. … Someone is getting the silent treatment at the next family function.