It's funny to read the reviews by people who are surprised that Georgie is living with Mandy's family. At the end of Young Sheldon, adult Sheldon explained after his father's death his mother Mary sells the family home, he leaves for CalTech, Missy lost control, and Georgie & Maggie don't last.
Mary, Mee Maw & Missy guest starring was announced by Annie Potts and Regan Revord on their official Instagram pages awhile back, so that was no surprise. Note they are Guest Stars not regular cast members, which only drives home to the audience that Georgie & Mandy's new home is with her family. If they stayed with Mary it wouldn't be any different from Young Sheldon; new premise, new show, new title. I liked the episodes and can't wait to see more!