The Daily Mail reports Giggs, formerly regarded as the model professional and committed family man, found his reputation further sullied by tawdry details of the relationship with his 28-year-old sister-in-law Natasha.
He had been cheating on his wife Stacey with her before she began dating his younger brother Rhodri, a Sunday newspaper claimed.
It said Giggs slept with Natasha the day after his eight-year-old daughter was born, and again the day after the arrival of his four-year-old son. The affair was alleged to have continued until April 9.
To underline 37-year-old Giggsâ€™s folly in taking out his notorious injunction over an alleged fling with former Big Brother housemate Imogen Thomas, his sister-in-law said it was this which spurred her to break her silence.
"When I found out he was cheating with Imogen too, I was really hurt," she told the News of the World through a â€˜friendâ€™.
"I know that sounds really strange, but he wasnâ€™t just cheating on Stacey, he was cheating on me too. Our relationship was just about sex to him."