[Undertaker]

Arata's brother? I don't remember he has a brother. I think you mistake the glasses guy who is just his practice partner in the club and used to playing Karuta with him when he was a kid.

If it's him,

Spoiler for Arata:

It has already been shown that Arata, even at last year's level when he just returned active, was a match for him. He was the challenger to Meijin 2 years ago which would put him around the same level as Tsuboguchi (from Chihaya and Taichi's club) and the guy that beat Arata last year in 4th round and defeated Tsuboguchi to be last year's Meijin challenger (the guy who was being described as a tank). Arata's from last year is already at or close at that level base on his returning match against Tsuboguchi in volume 7 or anime episode 21.

There is no doubt the current Meijin is the strongest, but he and Shinobu were league above others. This match shows that Arata has recovered to the point that he joins that pair. Though it should still be clear that even between the two Meijin Suo is clearly a league above Shinobu and his insistence on playing her might simply because there are no other players even worth his time.

But that would probably change after next chapter.