What gay joke are you talking about? She just assumed he was gay because he never showed interest in girls. Her being a mean sibling and saying it like that is different than making fun it.
That ending was the only way to end it properly. This could be like a prequel to a proper drama for Jisoo. Someone please give this beautiful man a main lead role!
I guess it's for the best, but still... Saiko&Otaro :'(