The Spanish word 'yerno' (meaning 'son-in-law') comes from the Latin word 'gener' (also meaning 'son-in-law'). The word evolved through several stages: first, the accusative form 'generum' in Classical Latin, then to 'genru' in Vulgar Latin as the ending simplified. In Old Spanish, it became 'gerno', and finally the initial 'g' sound shifted to 'y' to give us the modern Spanish 'yerno'. This type of sound change from 'g' to 'y' is actually fairly common in the development from Latin to Spanish, especially before front vowels like 'e'.

While 'yerno' has a fairly straightforward evolution from Latin to Spanish, it stands somewhat alone in modern Spanish without many common related words. This is because it comes from a specific Latin kinship term that maintained its narrow meaning of 'son-in-law' throughout its history.

Unlike some Spanish-English word pairs, 'yerno' doesn't have any closely related English cognates. English got its word 'son-in-law' from Germanic roots rather than Latin ones. This is a good example of how different languages can use completely different word roots to express the same family relationships.