Complex sentence: Lrrr and Ndnd are both annoyed that the creatures are destroying the planet, especially since "the Johnsons" are coming to dinner, who turn out to be Kang and Kodos.
Split into simpler sentences: Lrrr and Ndnd are both annoyed that the creatures are destroying the planet, especially since "the Johnsons" are coming. "The Johnsons" turn out to be Kang and Kodos.