nephew
侄 centers on a nephew, a specific male relative in the younger generation.
侄 combines 亻 (person) with 至, which likely contributes the sound. The character is used for nephew, but the exact historical development of the component relationship is uncertain.
The person radical 亻 beside 至 suggests a younger relative arriving: a nephew coming to visit.
For シツ, imagine a nephew sitting quietly in a room: sits -> シツ, and the nephew is the person sitting there.