Younger-sister [topic-は] own [possessive-の] all [possessive-の] belongings [object-を] cherish.
大切にする ('to cherish, to take good care of') — 大切 ('important/precious') + にする ('make into that state'). 持ち物 ('belongings') is 持つ ('hold') + 物 ('things'). すべての ('all of') modifies the noun.