Kate also reveals which A-lister made a very shocking entrance to one of her parties.
Called Truth or Shot, the two siblings -- the children of Goldie Hawn and Bill Hudson, raised by Hawn's longtime partner Kurt Russell -- were asked to answer a number of revealing questions by the host, or take a shot of vodka to avoid answering the Q at hand.
First, they were asked to reveal something illegal they did in high school. "Make fake IDs," said Kate, who added that she also then used them.
Oliver looked ready to spill more, but Kate talked him out of sharing his bad boy past. "I'm afraid people will think poorly of me," he said, before throwing back some alcohol.
They were then asked to share the last time mom Goldie embarrassed them in public.
"She just talked about the first time I told her I had sex," said Kate. "To someone, in front of me. That was embarrassing to me. I was like, 'Mom, you don't tell people about me losing my virginity!'"
Next: "Name a famous person you've kissed that we don't know about." Neither of them wanted to share that secret, so they simply clinked shot glasses to throw back another one together.
Lastly, they were asked whether either of them had smoked marijuana with their parents. They both had.
In their full interview with Ellen, the siblings talked about expanding their families. Right now, they both have three children each, with Oliver saying he's "done" having more kids.
"I don't know if I'm done yet," said Kate, who welcomed youngest daughter Rani in October 2018. "Right now, Rani's at the age where I'm like, 'I want another baby.' But once she gets like 4, you're like, I feel like my life is kinda back a little bit, you're kind of in a groove. There's a window."
"My window has never been open for one at all," joked Ellen. "I haven't had a window. I do think they're adorable and I like playing with them. Playing with children is fun and then, bye bye."
The two also talked about their relationship getting stronger once they got older and began partying together, throwing bashes at their parents' home whenever they were out of town.
"Once we were old enough to like each other's friends, things were real good," said Kate. "We were naughty when our parents would leave town and we always had like parties. Mom would find out about it, if a lamp was broken it was not good."
"She would invite industry people and I would invite the riff raff," added Oliver, "and she would always get pissed off at me."
Kate then recalled a time an A-lister made a very surprising entrance to one of their soirees.
"I'm standing at the door, making sure people who aren't supposed to be at the party aren't coming in. There's like 400 people at my parents' house," she began. "I see someone scaling, literally scaling an 8 foot gate at my parents house. I'm freaking out. They come off and do a back handspring thing. I'm about to yell at this guy and it's Tom Cruise."
"I was like, 'Oh, I didn't realize you were coming to our party,'" she added. "He's like, 'I heard there was a party.' Yeah, Tom, come on in! He is Mission: Impossible."