I stayed at my uncle's house during the summer.

Literal

I [topic-は] summer [possessive-の] during, uncle [possessive-の] house [at-に] stayed.

夏の間 ('during the summer') — 間 (あいだ) marks the duration. おじさん ('uncle') — the informal/familiar term. 滞在する (たいざい, 'to stay, sojourn') is more formal than 泊まる.