Complex sentence: He gave one of the twin swords, Bakuya, to his little brother, Ryuuki; out of all his other brothers, Seien was the only one who treated Ryuuki with kindness.
Split into simpler sentences: He gave one of the twin swords, Bakuya, to his little brother, Ryuuki. Out of all his other brothers, Seien was the only one who treated Ryuuki with kindness.