The Spanish word 'hermanastra' (stepsister) comes from combining the word 'hermano' (brother/sibling) with the pejorative feminine suffix '-astra'. The base word 'hermano' traces back to the Latin word 'germanus' meaning 'genuine' or 'natural', which itself derives from Latin 'germen' meaning 'germ' or 'sprout'. The suffix '-astra' adds a negative or pejorative connotation, effectively marking the relationship as 'not genuine' or 'not natural', thus creating the meaning of 'stepsister'.

This etymology reflects how the concept of step-siblings was historically viewed - as a relationship that wasn't considered as 'genuine' as blood relations. The development from 'germ/sprout' to 'genuine' to 'sibling' shows how the idea of natural growth became associated with blood relations.

The most common related Spanish word is 'hermano' (brother) and 'hermana' (sister), which are much more frequently used than 'hermanastra'. These base words carry the core meaning of sibling relationship without the pejorative step- relationship connotation. Other related forms include 'hermanito/hermanita' (little brother/sister) which add diminutive suffixes instead of the pejorative one.

While English 'brother' and 'sister' are not etymologically related to 'hermanastra', the English word 'germane' (meaning relevant or appropriate) comes from the same Latin root 'germanus'. The meaning evolved differently in English, focusing on the 'genuine' aspect to mean 'closely related' or 'relevant', while in Spanish it developed to specifically mean family relations.