The brief, past relationship came to light just weeks after Dannii, 40, announced in April that she was separating from partner Kris Smith, the father of her young son Ethan.
Minogue, 44, told Glamour magazine “my sister is going through a really difficult time” adding of the revelation: “It was a bit of a second blow after ‘Oh, my relationship has broken down.’”
Asked whether she thought that X Factor boss Cowell had betrayed her sister, a former judge on the TV show, by talking to biographer Tom Bower, she said: “Well, he doesn’t need to do any of those things. Or maybe he does... who knows? I don’t know the guy.”
Minogue said that her younger sister, who is in Australia, was “being pragmatic” since the split with her partner. “She and Kris have been amicable, and after heartbreak and disappointment, then it’s just organisation, concentrating on Ethan,” she said.