、。
Not only does she play tennis, she's also very good at skiing.
Literal
She [topic-は] tennis [object-を] play [not only-だけでなく], skiing [also-も] very skilled is.
~だけでなく~も is the standard 'not only X but also Y' frame — pairs two propositions with the second adding to the first rather than replacing it. The も on スキー reinforces the additive logic: 'tennis (already established) ＋ skiing (added).' Both スキー and テニス are loanwords, written in katakana as is conventional for foreign sports. 上手 (jōzu) here applies straight to the activity-noun スキー with も replacing が in this position; the も takes priority for marking the additive focus, even though the grammatical role of スキー would normally call for が.