Call it Jon & Kate Plus One Angry Brother.
In a new interview with E! News, Jason claims that his Reading, Pa., home was the rendezvous spot of choice for his 23-year-old sister and the 32-year-old Gosselin, both of whom have emphatically denied that their relationship has gone beyond the realm of platonic.
The elder Hummel is also fuming over his sister's claims that he has been making up salacious details after her headline-grabbing public outing with Gosselin.
"I am not a liar," Jason insists. "The rumors are true. It feels good to let people know that I am not the liar."
And despite the ramped-up tabloid attention and continued production of the Gosselins' TLC show, Jason says: "Personally, I do think they're going to continue seeing each other. I think they think they can get away with it.
"I hope she is not in love with him."
He also offers more details of the purported affair.
"He'd pick her up at least three times a week," says Hummel, beginning in January or February of this year.
"She told me she was having a friend over that night, and I was like, 'OK,' " he continues, recalling his first encounter with the married reality-show father.
"She's never telling me she's having anyone over and, lo and behold, a couple hours later, here's Jon Gosselin in my living room…I didn't like him from the start."
Hummel also admits that he videotaped and leaked footage of Gosselin leaving Hummel's home early in the morning because he was "sick…of seeing all these rumors going around when you know what's the truth.
"It's literally going on right in front of my face."
Hummel later recalled one of his final encounters with Gosselin.
"My sister says to me, 'Oh, Jon's going to get you tickets to the Phillies game. He has season tickets.'
"He goes, 'Yeah, I have season tickets behind home plate to the Phillies.' So I'm thinking to myself, Is this like my hush money right here?"
Meanwhile, the entire scandal is turning into a family affair for the Hummels, as Deanne's other brother, John, is also breaking his silence and backing Jason's account.
"[Jason] is telling the truth, and they paint him in a bad picture, and that's why I'm here—to let people know he's telling the truth," John tells E! News.
As for Gosselin, there's apparently no love lost.
"His wife is crazy," says John. "I guess I would cheat on her, too."
As it happens, Kate Gosselin's side of the story will no doubt be heard when the fifth season of the reality show premieres on May 25.
"My kids are the reason I do everything," she says in a solo interview in a sneak peak of the show.
Jon, meanwhile, seems to be doing his best to keep things in perspective.
"The world isn't about ratings and TV. It's about happiness and love. Compassion and intimacy. Family and God. I think we all got caught up in everything.
"Kate and I have obviously been going through a lot of stuff and discussing what's best for our kids," he said. "It's a crazy life, but it's our life."