Just combine your money and share the book.
Of course, you'd have to decide whose house it would be kept at, but it'd be the perfect excuse to visit each other.
He made the right choice, he scored major points with the girl. An artbook can be bought anytime but a chance at making an impression like that doesnt happen often, and it lasts as long as the memory does.
Just FYI, the brother and sister pairing was last saw at the "Test of Courage" chapter. They also appeared at Kyoko-chan manga as well at Chapter 21. Believe it or not, that was over five years ago since their last appearance.