son, my son
倅 centers on a son, specifically one's own son, used as a humble or familiar term.
倅 combines 亻 (person) with 卒, which likely contributes the sound. The exact historical development is uncertain, but the character is used for 'son' in a humble context.
A person 亻 beside 卒: imagine a father introducing his son as his humble successor, a young person who will eventually take over.
For サイ, picture a proud father saying, 'This is my son, sigh (サイ) of relief, he's a good kid.' The sigh sound cues サイ while the father beams at his son.