Janet Jackson's (search) former brother-in-law made a bombshell claim on a hip-hop radio station Friday — the sexy siren has a secret 18-year-old daughter.

The stunning allegation came from Young DeBarge (search), whose brother James DeBarge (search) was briefly married to Jackson, now 39, in the 1980s.

During an interview on Hot 97, Young said the busty star and his brother — a former member of another Motown family act — are the parents of "Renee," a daughter sent away in shame to live with the eldest of the famous Jackson siblings, Rebbie.

"James and the Jackson family kept everything real close, real tight," Young DeBarge said. "No one really knew how it was working out until things kind of surfaced."

Janet Jackson's rep Patti Webster didn't deny the claim Friday, only saying the singer didn't wish to comment. And the young woman is not happy about her star mom's refusal to acknowledge her, the uncle said.

"I think there's a little despondence," DeBarge said when asked about Renee's feelings for Jackson.

"She hasn't really revealed that, but I'm sure that there's some feelings there. It's only natural that you wonder about what is and how it's come to be that way."

Jackson was just 18 when she and James DeBarge hooked up in 1984. Their marriage was annulled the next year.

The couple eloped amid family furor over their relationship, but the marriage was annulled in March, 1985., with DeBarge's drug habit cited as the reason for the split.