Complex sentence: To escape death, Grampa moves into the Simpsons house, where the family lets him live in Bart's room, putting a further strain on the relationship until Mr. Burns bursts in to take the last key to the paintings' safe.
Split into simpler sentences: To escape death, Grampa moves in with Homer and Bart, and lives in Bart's room, putting a further strain on the relationship. This changes when Mr. Burns bursts in to take the last key to the paintings' safe.