Sean – played by Rob Kazinsky – is furious when he learns Jack Branning (Scott Maslen) is Amy’s real father.
The gripping storyline is due to come to a head over Christmas dinner at the Queen Vic, when Sean loses it as the paternity test results are revealed in a home-made cracker – and he confronts wife Roxy (Rita Simons), reports The Sun.
The furious and broken man plans to get revenge on his wife and Amy’s twisted grandfather Archie Mitchell (Larry Lamb) by kidnapping Amy and leaving Roxy fearing the worst for her baby’s safety.
But Sean’s sister Stacey (Lacey Turner) offers to help them find her crazed brother and calm him down.
An EastEnders insider said: “Stacey is the only one who’s ever been able to handle Sean. He’s a loose cannon and he’s utterly humiliated.
“While everyone else panics, Stacey works out what’s happened and it’s not long before she’s on his tail.”