Sydney Sweeney will be returning to the franchise, this time based on the second novel in Freida McFadden’s trilogy, and will also be directed by Paul Feig. Sweeney has already signed on to executive produce the sequel, and while a deal for her to star hasn't been finalized yet, Lionsgate is keen on her reprising her role as Millie.
As for Amanda Seyfried, a co-star in The Housemaid, she too has an executive producer credit on the sequel. Her character doesn't appear in McFadden's second book, but the possibility of Seyfried appearing in the sequel is still a possibility, as reported by The Hollywood Reporter.

The production of the sequel, by Hidden Pictures and Pretty Dangerous Pictures, follows the box office success of The Housemaid, which is now in theaters and has earned $75 million domestically and $133 million worldwide, with international territories continuing to roll out through January.
Screenwriter Rebecca Sonnenshine, who adapted The Housemaid, will return to adapt the sequel for producers Todd Lieberman, Feig, Laura Fischer, Carly Elter, and Alex Young. McFadden will also serve as an executive producer.
"It’s clear from both the global box office and from the outpouring on social media that audiences have responded strongly – and audibly – to the totally unique and truly theatrical experience of The Housemaid and want to know what happens next," said Adam Fogelson, chair of Lionsgate Motion Picture Group, in a statement. "We believed in these stories from the very beginning, and we are beyond excited to bring the next chapter of Millie’s story to life on-screen in collaboration with our outstanding creative partners Todd, Paul, Laura, Carly, Alex, and Sydney. The Housemaid’s Secret is another wildly thrilling book in Freida’s series that has captivated readers worldwide, and we look forward to translating it into a similarly rousing and riotous moviegoing experience."
Feig added: "It’s been thrilling to see audiences around the world fall in love with The Housemaid and the incredible work of our talented cast and crew. We’re lucky that Freida McFadden has already extended Millie’s journey on the page, and that we get to work with Rebecca Sonnenshine and Lionsgate to bring this next story to audiences."
The sequel will be overseen by Lionsgate’s Chelsea Kujawa, who also oversaw the first film and brought the project into the studio alongside Maria Ascanio. Robert Melnik negotiated the deals for Lionsgate.