son-in-law
聟 centers on the family relationship of a son-in-law: a man married to one's daughter.
聟 is a variant form of 婿, combining 知 (know) and 耳 (ear), possibly suggesting a son-in-law as someone who listens and understands the family. The exact historical development is uncertain.
A son-in-law is someone who knows (知) how to listen (耳) to his new family.
For セイ, imagine a son-in-law saying 'say' (セイ) the right thing to his in-laws, using his knowledge and ears.