Sharon, played by Letitia Dean, makes her long-awaited Albert Square comeback in a fortnight's time - arriving on Phil Mitchell's doorstep in desperate need of his help.
Although Sharon has interrupted his and Shirley's engagement party, Phil agrees to put his former flame first as she explains that she has run out on her wedding day and needs him to help her get her son back.
Asked whether Shirley feels threatened by Sharon, Henry told All About Soap: "Oh, yes. Shirley does trust Phil, but she knows how much history he has with Sharon.
"Shirley and Sharon are complete opposites, so I think she finds it hard to believe Phil would choose her over Sharon, when she knows how he used to feel about his ex. She's worried that old spark will reunite."
While Phil is away helping Sharon, Shirley heads over to see Denise Fox - and is rocked to the core to spot the murder weapon which was used to kill Heather.
Henry continued: "[Shirley and Denise] have a heart-to-heart about Carly, but then Shirley spots the photo frame on the fireplace - she's sure it's the one that the murderer used to kill Heather. She wants to make sure she isn't imagining it. She demands to know where Denise got it from."
"I think her first reaction will be to go to the police, but they've not been very helpful so far and have even criticised her for getting involved in the investigation," she added. "So I think she could change her mind and decide that she wants to catch the killer herself."
EastEnders will air seven episodes in the week commencing August 13, as Sharon makes her return and Shirley finally discovers the truth about how Ben Mitchell killed Heather.