mother
妣 centers on the concept of a mother, specifically a deceased mother in classical usage. It is a respectful, somewhat archaic term for one's late mother.
妣 combines 女 (woman) with 比, which likely contributes the sound. The character originally referred to a deceased mother, contrasting with 考 for a deceased father.
A woman 女 compared 比 to no other—your late mother is irreplaceable.
For ヒ, imagine the sound of a soft, respectful sigh: 'He...' when remembering a late mother. He -> ヒ.