JT's Gran, Sadie Bomar, accidently let the details slip to Star magazine while explaining why she and Justin's Grandfather wouldn't be able to attend.
She told the magazine: "Unfortunately, we won't be going to Italy with my daughter [Justin's Mum] in October."
She continued: "My husband, Bill, has had problems with his heart for a while now and is undergoing some major heart surgery soon."
"Bill also suffers from dementia, making it tough to care for him."
[Related story: Jessica Biel v Kate Beckinsale in fash off take 1]
[Related story: Justin Timberlake and Jessica Biel marry in secret?]
The showbiz couple had been dating for five years before becoming engaged last Christmas.
Neither Justin nor Jessica has discussed the details of the upcoming ceremony, with Jessica admitting last month that she hadn't had the time to plan anything.
She said: "It's very exciting but it's hard to find any time for it."
Well if Justin's Gran's claims are anything to go by it seems the couple have indeed found the time to wed- and it's next month!
It was rumoured last month that the couple had wed in secret in Wyoming, America, the location where Justin had allegedly got down on one knee and popped the question to Jess.