This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 宑 is uncertain. It visually combines 宀 (roof) and 井 (well), but the original meaning and component relationship are unclear.
With no clear meaning, picture a roof over a well: 宀 over 井. The image is of a covered well, but the kanji's meaning remains obscure.
For セイ, imagine a covered well that is a sacred (セイ) place: the well under the roof is treated as holy, and セイ echoes the sound.