homologue; homolog
Technical term in chemistry and biology for a compound or structure that shares a common ancestry or structural similarity with another.
メタンとエタンは同族体の関係にある。
Methane and ethane are homologues.
同族 is a broader term meaning 'same family' or 'same group', while 同族体 specifically refers to a homologue in chemistry or biology.
Compound of 同族 (same family) and 体 (body, form), likely a calque from Western scientific terminology.