The couple met on the set of US hit Revenger - where Emily starred as Emily Thorne and Josh played her boyfriend Daniel Grayson.
After Revenge came to an end in 2015, Emily has taken on a variety of other roles, including Sharon Carter in the Captain America movies. Her future sister-in-law Scarlett Bowman - who previously starred in British TV soap "Hollyoaks" - logged on to her own Instagram account to express her excitement that her younger brother Josh is getting hitched.
In a case of life imitating art, the pair's Revenge characters, Emily Thorne and Daniel Grayson, were actually married during season 3 of the show in an over-the-top ceremony.
Josh portrayed Jack The Ripper in the ABC drama Time After Time that was cancelled in March after five low-rated episodes. On her ring finger was a giant sparkling diamond, and the bride-to-be captioned the snap with a simple red love heart emoji.
When discussing her personal life and her career, she said, "Josh and I keep it all very separate".
"I have a wonderful love in my life", she said, adding: "Trust is the most important thing in any relationship, because everything falls into place after that". It's just really amusing in the moments when we're playing these absolutely absurd things. "I find myself dreaming about [having children] all the time", she told the magazine. "In your late 20's, you start to realize you can't call yourself a baby anymore".