It was then that it struck me that I could use a family tree to trace who those 'strangers' in/at my grandparents' house were.
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KooyeenHi Yoong,Hi Kooyeen
It was then that it struck me that I could use a family tree to trace who those 'strangers' [in/at] my grandparents' house were.
in - inside. To me, it gives a feeling of "intruders in the house", so "strangers" could be understood with a more negative meaning.