Bill (Nick Kroll) has been overshadowed by his brother Robbie (Adam Scott) since a childhood accident resulted in Robbie becoming blind.
As for the brother's feelings toward Bill, Robbie gives a big speech after a marathon-length run.
Bill has been supporting his brother's efforts, which involve displays of physical endurance to raise money for a charitable organization for the blind.
In the process, Robbie has become a local hero, while Bill works at a dead-end job at a printing shop when he isn't helping his brother train for the next event.
When she corrects him, he's embarrassed, and wanting to save him from that embarrassment, Rose kisses him. Bill, of course, now has another reason to be jealous of his brother.
The comedy here goes deeper than the awkward situation or the ways in which Bill and Rose try to hide their recent entanglement from Robbie.They are inclined to lie—to each other, in order to keep their relationships relatively nice and calm, and to themselves, in order to keep up the other lies. These are fairly decent people, and they lie because they want to stay decent—or at least to keep up the appearance of being good people.