THEY'RE ALL GAY WITH EACH OTHER.
anyway, jokes aside.
well, tbh, i really don't know. sexual orientation is still a sensitive subject in Japan. but i think you also have to look at cultural differences too. Japanese ideals of masculinity & let's say American ideals of masculinity are very different. i suppose it wouldn't be weird to see them being really friendly with each other, but to Americans, it would imply something else (gay behavior). not to mention male looks in general, especially with Johnny's.
but concerning Yassu...sometimes he's very girly. LOL. and he has a bizarre fashion sense. i could see why your sister thinks he's gay, but tbh, i wouldn't be surprised if he wasn't. lol.