he can't play on fruit snack guitar strings with fruit snack fingers!
the younger brother has escaped this mortal coil to a fruity delicious coil, but the older brother will know naught but the eternity of his own silent soul
If you assume that the younger brother wanted to be transformed in exactly this way, the whole exchange is a fantastic manipulation.
He was the first to introduce the idea of body modification to the battlefield, but he downplayed his attacks so that the brother never shares the bliss of a candy body.