Last month, Kelly-Marie's younger brother Bob headed off to Peru after hearing that his sibling was having relationship troubles, but both will be returning later this year.
Show bosses have confirmed that Kelly-Marie and Bob will make their screen comebacks in September, but Pieraccini has already returned to filming at the programme.
Speaking of Kelly-Marie's reappearance, the actress explained: "I'm really excited about coming back and being with all the Adams again, and of course seeing the cast again.
"It's been three years since I last played Kelly-Marie and a lot has changed with her - but she's still the same girl at heart and will be lots of fun to play again."
Meanwhile, Stephen Purdon, who plays Bob, said: "I can't wait to get back to work especially as I will be working with Carmen. She really is just like my big sister and we have always had great fun working together."
Purdon took a break from filming at River City earlier this year due to illness but is now shooting scenes again. Discussing his time away, he added: "I'm now beginning to feel back to myself and want to thank everyone for their support and wishes."
River City's executive producer Gaynor Holmes commented: "It's great to see Stephen back at work and we're delighted to be welcoming Carmen back to the show too. It's fantastic to see the return of two much-loved characters at a time when the show is continuing to go from strength to strength.
"Bob and Kelly-Marie have always been very popular with the audience and they are bringing with them an exciting storyline which is set to shake things up in Shieldinch."
Lenny Murdoch, played by Frank Gallagher, is also returning to River City.
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