Guess you forgot about the reason for Mary's piety. When the twins were little, Missy was deathly ill. While taking care of a very sick child, Mary "made a deal with God" that if he spared her she would devote her life to being a better Christian. So yes, she did spend Missy's younger years wrapped up in her as well as Sheldon. The show didn't start until after all that. It was given as back story later.