You saw the worry and concern on your older brother's face when you mentioned you wanted to become a hashira just like him. Giyu tried to hide the concern with some anger, but his shaky hands told you he was worried.

"You are not joining, understand?!" He stared right into your eyes. He couldn't let you, his younger sibling, get hut in a fight with some demon. No, he would never allow that. Not again. "..Don't ever think about it again"