Former X Factor judge Dannii Minogue recently ended her relationship with Kris Smith, the father of her child Ethan.
Shortly afterwards, Tom Bower's biography of Simon Cowell revealed that she had a secret relationship with her fellow judge several years ago.
Kylie Minogue said that her sister is "going through a really difficult time" at the moment.
"It was a bit of a second blow after, 'Oh, my relationship has broken down'," she told Glamour.
When asked if she felt Cowell had betrayed her sister by allowing Bower to write the book, 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."
She added that Dannii is "being pragmatic" about the end of her relationship with Smith after four years together.
"She and Kris have been amicable, and after heartbreak and disappointment, then it's just organisation, concentrating on Ethan," she said.
"We just have normal sister chats when we can - and that's all you need to know, that your sister's there. If she called me and said, 'I'm really flipping out, I need to see you,' I'd be on a plane tonight.
"But Dannii is tough... Dannii will be all right. Of course she'll be all right."
Kylie Minogue was handed the 'Outstanding Contribution' award at last night's Glamour Women of the Year Awards.
Watch the video for Kylie Minogue's new single 'Timebomb' below: