It's nice to not tattle on your brother.
Joan is the only one who knows her brother is gay, so she's unsure how to respond when her parents ask her who her brother is dating.
Joan wants to do whats best for her brother.
Joan refuses to answer and instead urges her brother to open up to their parents.
Joan decides that honesty would be best and tells her parents that her brother is dating a guy.
Joan is able to be the support her brother needs to come out to their parents.
Joan has a falling out with her brother when he realizes that she took that choice away from him.