、。
My sister can already read books well.
Literal
Younger-sister [topic-は] already, books [object-を] skillfully read thing [subject-が] can-do.
もう ('already') signals a milestone has been reached. 上手に ('skillfully') is the adverbial form of 上手. ~事が出来る is ことができる written with kanji 事 and 出来る — same meaning as the hiragana form.