Singers Diana Krall and Elvis Costello are expecting twins , Diana Krall tells PEOPLE. "Today I’m at six months," Krall, who is promoting her new album, From This Moment On, said Wednesday. "And you know, I’m having twins! I’m not kidding!" Krall, 41, and Costello, 51, know the sex of their twins – due in December, around the couple’s third anniversary – but they aren’t telling. "We know, and everyone’s been asking me, but we’re keeping that private because we just want to have something for ourselves," Krall said. Krall said she’s genetically disposed to a double delivery: "I have twins on my mother’s side as well."