。
I wonder if she'll be able to finish it today.
Literal
She [topic-は] today it [object-を] finish-doing [potential-ことができる] [I wonder-だろうか].
し終える is a compound verb: する's masu-stem し plus 終える ('finish') = 'finish doing.' Japanese builds compound verbs prolifically, especially with secondary verbs like 終える, 始める, 続ける, 直す — combine any masu-stem with these to layer aspectual meaning. ~ことができる is the formal/written counterpart of the bare potential form (し終えられる); it sounds heavier and tends to appear in writing or careful speech. The trailing だろうか is a sentence-final 'I wonder,' marking the whole statement as a self-directed question — softer than questioning the listener.