nephew
甥 centers on a specific family relationship: a nephew, the son of one's sibling.
甥 combines 生 (life, birth) and 男 (man, male), suggesting a male relative connected by birth. The exact historical development is uncertain, but the component choice fits the meaning of a nephew.
A male (男) relative born (生) from your sibling is your nephew.
For セイ, imagine your nephew saying 'say' as his first word: say -> セイ, and the proud family celebrates the new life (生) of this male (男) child.