In a recent interview, Dermot Mulroney confirms that a sequel to The Family Stone is definitely happening and the whole cast is returning. He also comments on the script and reveals the sequel's title: The Families Stone.
"Everyone’s up for returning. I can be on record," Mulroney said on the show. "I’ve read it, I can’t even process yet the depth of it, especially in light of losing Diane this year. We didn’t remain in touch [as] friends but she was so impactful on all of our lives in that mother part in that life-altering movie."
Source: Entertainment Weekly