This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 宷 is uncertain. It visually combines 宀 (roof) with 釆, but the original semantic and phonetic roles are unclear.
With a roof 宀 over a 釆 shape, imagine a covered space where something is carefully examined or distinguished, but the true meaning is obscure.
For シン, picture a thin (シン) beam of light shining under the roof 宀 to inspect the 釆-like object: thin -> シン.