Simplify the following text for a Grade 7 native English-speaking person.
Original: When Casimir died in 1633 , his brother Ernst of Saxe-Eisenach ruled in personal union over Saxe-Coburg until his death in 1638 .
Simplified: When Casimir died in 1633 , his brother Ernst of Saxe-Eisenach both countries until he died in 1638 .