son-in-law
壻 centers on a man who marries into a family: a son-in-law.
壻 combines 土 (earth) with 胥, which likely contributes the sound. The exact historical development is uncertain, but the character is a variant form of 婿.
The left side 土 suggests a foundation, and the right side 胥 looks like a person settling in. Picture a son-in-law becoming part of the family's foundation.
For セイ, imagine a son-in-law saying 'say' yes to joining the family: say -> セイ.