Bosses at the BBC soap have cast actress and singer Polly Perkins in the role of Rose, who has had little contact with Dot for most of their lives.
Animosity between the siblings began many years ago when Rose embarked on an affair with Dot's first husband Charlie.
However, after suffering a bout of hypochondria later this year, Dot - played by June Brown - will decide that it's finally time to put things right and track down her sister.
Billed as "flighty, fun and not one to age gracefully", Rose is very different from Dot and is not pleased to see her after so many years.
However, insiders have hinted that there could be another reason why Rose is uneasy about the unexpected reunion.
Perkins is best known for her role as Trish Valentine in Eldorado. She has also appeared on screen alongside Brown before as they both appeared in BBC drama Rules of Justice in the 1980s.
Speaking of her casting, Perkins commented: "I am thrilled to be joining the cast of EastEnders, the show is a real British institution with an extraordinary creative team. I'm really looking forward to working with June again, who I have been friends with for over 30 years."
Fans have seen Dot left heartbroken this week as her beloved husband Jim was moved into a care home because she could no longer look after him on Albert Square.
Her new storyline will air in the summer.
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> Read our recent exclusive interview with June Brown, who plays Dot