。
We're staying at our uncle's place.
Literal
We [topic-は] uncle [of-の] house [at-に] are-staying.
叔父 (おじ) is a father's or mother's younger brother; 伯父 (same reading) is the older brother — kinship terms distinguish birth order. 泊まる ('to stay overnight, to lodge') takes に for the place. ~ています marks the ongoing state of the staying.