The reason Page One was considered in the spoiler threads to be Ulti's sister is because in the Korean scanlations he refers to her as "Nuna"; if you ever read a Korean Webtoon, you will know that that term is used to describe a bigger sister, but also an older female figure (in other words, Onee-san). So while this doesn't confirm 100% that they are siblings, it does confirm that Ulti is older than Page One, making him most likely the youngest of the 6
Its not confirmed that they are Siblings