Hmm... so were the cookies hand made and he lied, or did he only give hand made to his sister?
The handmade cookies he made were terrible and he didn't want to give those to Sasaki, so he went out and bought some for her instead. Probably figured his sister wouldn't mind one way or another because well, family. His logic is understandable in that he wanted to give Sasaki something she would be able to enjoy, but he underestimated the sentiment of receiving a gift full of the hard work and dedication from someone you
love care about. So now she's jealous and sulking.
Meanwhile his sister has no idea he went out and bought cookies for Sasaki, and assumes Sasaki got homemade cookies too. So big sis is unintentionally bragging because Sasaki thinks he put more effort into his gift for his sister.