。
She is used to cooking.
Literal
She [topic-は] cooking [object-を] doing [to-のに] is-accustomed.
~のに慣れている means 'to be accustomed to doing X.' The の nominalizes the preceding verb phrase, and に marks it as the target of 慣れる ('to get used to'). ている marks the resulting state of having become accustomed.