Keep in mind tho, Page One calling Ulti nuna (big sis) doesnt necessarily 100% mean that they are siblings. However, it does certainly confirm that Ulti is older than him. Nuna/Noona is used by male to call their older sister/women older than him.
Also, I opened reddit and saw that apparently the word "grandpa" is being thrown in the conversation among the tobbi roppo in french/english scanlations.
I definitely think it's a mistranslation, because the tobi roppo (Black Maria, Sasaki, Ulti) all called Page One as "Pe-ttang" (korean). It is a very cute way to call someone's name with -chan/tan ending, like "Pe-chan/Pe-tan" in japanese. Not sure why the word grandpa is being brought up. Page One is definitely the youngest, all of them are teasing him.
Page one even said " 페땅이라고하지마!!" STOP CALLING ME PE-CHAN/TAN!!
Page One's full name in korean is 페이지 원. They called him 페땅 (Pe-ttang). It's probably going to be translated as Page-chan/tan in the officials.