Rhodri Giggs branded his sibling an "embarrassment" and claimed that the 38-year-old "never apologised" for almost destroying his marriage to the Celebrity Big Brother star.
"Ryan's a great footballer but he is nothing as a man," Rhodri told The Sun. "He is a worm, a weasel and a bottler. I always thought he was a great person, but now I know he's not.
"He's an an embarrassment, he's a bottler. He's never apologised and its good to know he's ruined my life over sex.
Rhodri said that he was hoping to rebuild his marriage to Natasha, describing the 29-year-old as his "whole life".
"The hardest thing was visioning them together," he said of the affair. "When someone's been your hero most of your life and then you find out he's cheating and lying and sleeping with your wife behind your back - its hurtful.
"But I love Natasha. She's my wife. I want to try and salvage it if I can, I have got a good family, a good home and I just think she just got manipulated by Ryan at a young age."
Natasha, who was evicted from the Celebrity Big Brother house yesterday (January 13), expressed hope last month that the former couple would reconcile.
"I sleep alone in the bed I used to share with Rhodri and wish he was back in it with me," she said. "I hope time can heal the pain I've caused. This has been the worst year of my life and I know I only have myself to blame for the way things turned out."
Details of Natasha and Ryan's affair emerged in June following the super-injunction scandal surrounding his fling with ex-Big Brother contestant Imogen Thomas.