、。
She can play not just the piano but the violin too.
Literal
She [topic-は] piano [topic-は] of-course-thing-of, violin [also-も] is-able-to-play.
~はもちろんのこと is a polished alternative to ~だけでなく for 'not only X but also' — literally 'X is needless to say (a thing of course),' framing X as so obvious it barely merits mentioning. The は after ピアノ is contrastive, narrowing focus before sweeping out: 'as for piano, that goes without saying — and on top of that...' The second clause uses も to add the violin to the implied list. Notice the potential 弾ける ('can play') — Japanese moves freely between describing what someone does (弾く) and what they're capable of (弾ける).