Sweetin, who is 19 years Stamos' junior, responded to a fan who commented on a throwback Instagram photo she posted Sunday. The snap showed the actress and Stamos back when they were "babies" and was intended as a happy birthday post for the 55 year old.
"Happy Birthday, Uncle J!" she captioned the pic. "My, my... we've made quite the memories these last 30+ years! I can't believe you're about 10 years younger than I am currently in this photo!! We were babies! (Well, I actually was... haha) Love you so much @johnstamos !!!"
A user commented on the photo and asked, "How can you still say uncle to him when it's been reported that you slept with him?! That's just nasty."
Sweetin, 36, shut down the troll, and made it clear the claim was categorically untrue.
"Hahahah...um...what?!" she wrote in the comment. "Yeah...that's def not true.”
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Sweetin and Stamos starred together on the original "Full House" series -- which ran from 1987 to 1995 -- alongside Candace Cameron Bure, Andrea Barber, Bob Saget, Mary-Kate and Ashley Olsen and more. The duo, who played Stephanie Tanner and Uncle Jesse, respectively, reprised their roles in the 2016 Netflix spin off series "Fuller House," which currently has three seasons.
Stamos welcomed his first child -- son Billy -- with wife model Caitlin McHugh this past April.