It's only from a starting point that she is 'someone's sister'. I refer to that other guy as "Mayu's brother"
Yuki Ishikawa won the awards of best OH in both of the last 2 World Cups, and had an important role to lead JPN to get their best result in the 2019WC. IMO he deserves better credit than being said as “someone” or “that other guy”.
I like Mayu Ishikawa and I even think she has the best attacking skills among players at her height in the world. But at this point there’re still a lot to improve in her game. I do hope she can improve her reception if she wants to be a world class OH like her brother. If she could achieve the level of success of Yuki, she’d definitely be the role model of short player to the world.