The most explosive blow-by-blow details are revealed by Deanna's older brother - and roommate - Jason, who tells Us Weekly that he is speaking out now to set the record straight and to protect his naive sister.
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"She's a nice girl, not a homewrecker," Jason tells Us Weekly. "He is a bad liar. This isn't healthy for her. But she is refusing to help herself, so here I am trying to help her myself. I hope this clears the air."
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After Jon and Deanna met in mid-January at Chill Lounge in Reading, Pennsylvania (they sent each other $3 shots), Jason says Jon -- who has twins, 8, and sextuplets, 5 -- began relentlessly pursuing his sister. By mid-February, he was coming over to their Reading house.
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On April 26 - while his wife Kate was traveling - Jon had Deanna over to sunbathe in his family's front lawn (see photo, left). This was just seven days after they had first been photographed together at Legends Lounge at 2 a.m. in pictures that show he is not wearing his wedding band.
At the Hummel house, Jason says Deanna and Jon would "pretty much stay locked away like two teenagers. It was weird. He's a grown man."
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He also adds, "A lot of the time, it was pretty, um, gross listening to her, you know, um — how do I say this? The walls are thin. Let’s just say that. I mean, no one wants to hear his sister having sex, let alone with a married dude who's, like, almost twice her age and who has eight kids and a maybe-crazy wife. Ick. Nast."
SEE A VIDEO OF JON GOSSELIN SNEAKING OUT OF DEANNA HUMMEL'S HOUSE
For more explosive details on the affair -- including Gosselin's bizarre courtship style, their first cheap dinner date, his reckless behavior, how Deanna got in trouble at work after the Legends' photos appeared, the gift he bought for Deanna while he was on a trip with Kate, and how Deanna believed Jon was going to leave Kate for her -- pick up the new issue of Us Weekly today!