Is Onishi Kenya the brother of Onishi Ryuhei?
It seems a strange coincidence that two strong young players, born within five years of each other, with the same surname wouldn't be related.
At least when the name isn't super common, like "Shin", "Park", "Cho", "Wang (O)"... no other pros are called Onishi.
According to this data, there are about 177,200 Onishi in Japan. So it's a pretty common surname (of course, Sato or Yamada is more common).
Also, if they were brothers, Nihon Ki-in would describe relatives in their profile page, just like Mannami Kana ( profile) and Mannami Nao ( profile). In addition, Japanese Wikipedia has no description about brothers.