Complex sentence: Carl and Ellie eventually get married and grow old together in the restored house, and they planned to have children, but Ellie was diagnosed as infertile, so Carl wanted to fullfill their promise of travel to South America.
Split into simpler sentences: Carl and Ellie eventually get married and grow old together in the restored house. They had planned to have children, but discover Ellie is infertile, so Carl determines to fulfill their promise of travel to South America.