。
She didn't expose her skin to the sun.
Literal
She [topic-は] skin [object-を] sun [to-に] did-not-expose.
さらす ('expose') takes the standard 'X-を Y-に' frame: what is exposed is marked with を, what it is exposed to with に. The same frame applies to 当てる ('apply, hit against'), 向ける ('direct toward'), and many other verbs of orientation.