Complex sentence: Carrie's fanatically religious mother, Margaret White (Patricia Clarkson), who considers menstruation a sign of sexual sin, locks Carrie in her "prayer closet" as punishment.
Split into simpler sentences: Carrie's fanatically religious mother, Margaret White (Patricia Clarkson) is there. Margaret considers menstruation a sign of sexual sin, and locks Carrie in her "prayer closet" as punishment.