It is onii-san in the raws, that's why I translated it as big bro. That's also why the old man is flustered and talked about his flattery on the next panel
They're your translations. You can do them however you like and I thank you for your efforts either way. Translating words with contexts that don't match or that have no direct translations must be very difficult. (I'd feel sorry for someone trying to translate Schadenfreude to English instead of using a TL note...)
(Not trying to debate. Sorry. I just dislike Big Bro and Big Sis so much...)
I'm just stating that my preference is to leave those terms as they are. If they're going to be translated then why not translate senpai to senior and kouhai to junior? Calling a stranger Onii-san would be respectful in Japanese but calling them Big Bro in English wouldn't be. The respectful context is lost so, IMO, "onii-san" with a TL note for the context is better than "big bro".
All of the other terms (uncle, aunt, grandpa/grandfather, grandma/grandmother) are much harder especially the last two because there are times when in American English you might call an unrelated person grandpa to their face in a respectful way.