。
Besides the guitar, she also plays the piano.
Literal
She [topic-は] guitar [of-の] besides piano [also-も] plays.
~の他に ('besides, in addition to') is a noun-attaching connector — pin it to a noun to add 'besides X, also Y.' The companion も in the second clause (ピアノも) marks 'piano' as the additional item being added to the implicit list. This X の他に Y も... pattern is the standard frame for additive listing: 日本語の他に英語も話す ('speaks English besides Japanese'), 本の他にCDも買う ('buys CDs besides books').