。
She's currently staying at her uncle's house.
Literal
She [topic-は] now uncle 's house [at-に] is-staying.
滞在する ('to stay, to reside temporarily') is a Sino-Japanese verb for short-to-medium-term residence — visiting, on a business trip, abroad for a season. Compare 住む ('reside long-term'), 泊まる ('stay overnight'). おじさん in kana can mean either uncle, but the kanji split is sharper: 叔父 is the parent's younger brother, 伯父 the older brother. In speech and casual writing, おじさん covers both — and can also broaden to 'an older man' generally, depending on context.