。
She's used to sitting.
Literal
She [topic-は] sitting thing [to-に] is-accustomed.
~に慣れる ('get used to X / become accustomed to X') marks the thing one adapts to with に. Pairing it with the nominalizer こと converts the verb 座る into 座ること ('the act of sitting') — a noun phrase that can take に. The progressive ~ている lands as a state: she's settled into being accustomed, not in the process of getting there. The bareness of the sentence makes it slightly odd in English without context — perhaps about long meditations, formal etiquette, or a body that's adapted to a particular kind of sitting.