Kat, played by Jessie Wallace, will deal husband Alfie an emotional blow when she becomes close to his cousin, The Daily Star reports.
Viewers have witnessed the breakdown of Kat and Alfie's marriage after Kat's affair with Derek Branning was uncovered late last year.
In scenes to air on the BBC soap next month, Kat will be seen getting closer to Michael, the biological father of her son Tommy. Michael is also a single parent following the departure of his wife Janine.
An EastEnders source revealed that Alfie will be left devastated by Kat's choice of man.
They said: "Kat and Michael are both struggling single parents. He's had to take care of his little girl Scarlett ever since Janine fled Walford and has remained close to Kat these last few months.
"It was Michael who Kat first told she'd been cheating on Alfie. Although she didn't tell him who with but he's been keeping an eye on her. And with them both now single they kind of end up together. It makes sense. They're both lonely and there's always going to be something between them because of little Tommy.
"Michael has never really been a part of little Tommy's life, Alfie has brought him up and Alfie is his dad. But when Kat moves in with Michael it does hurt Alfie. He tries to put a brave face on it all as he's with Roxy now but Kat will always be the love of his life. It's going to be an interesting time for both couples."
Charlie Brooks, who plays Janine, is set to return to the soap soon following her stint on the last series of I'm a Celebrity...Get Me Out of Here!.
The source added: "Janine will return to find Kat living in her house with her baby and her husband. You can just imagine what her reaction is going to be. Kat is never one to shy away from any sort of confrontation but whether her relationship with Michael will survive remains to be seen. Janine coming back will certainly put it to the test."
An official EastEnders spokesperson said that they "could not comment on future storylines as we don't want to spoil it for our viewers".