descendant, border
裔 centers on the idea of a hem or edge, which extends to mean a distant descendant—someone at the far edge of a family line. The border sense is the literal edge, while descendant is the figurative edge of lineage.
裔 combines 衣 (clothing) with 冏, which likely contributes the sound. The original meaning is the hem or border of a garment, which later extended to mean a distant descendant—someone at the far edge of the family.
The top 衣 is clothing, and the bottom 冏 looks like a border or edge. Picture the hem of a robe trailing far behind, like a descendant at the distant edge of a family line.
For エイ, imagine an heir (descendant) standing at the border of the estate: heir -> エイ, and the hem of their robe marks the edge.