Phoebe and Kai are each other's best friends. Phoebe and Kai have never lived together, except for, after years of German litigation, the 30% of the time they were allowed to be with their dad. Now they are blocked from even that while the German courts sit on their hands, like they do with most fathers.
But apart from the sad back story, watching this video, you can see how much they enjoy each other.
The music is from a live concert of Donny and Marie Osmond and the Osmond Brother's singing: "He Ain't Heavy, He's My Brother" written by Bobby Scott and Bob Russell.