fratino

Fratino is frato plus -in-, the feminine suffix, and it is the standard illustration of how Esperanto builds family vocabulary from a handful of roots. The same machinery gives patrino from patro and filino from filo. The other half of the system is ge-, which combines the sexes: gefratoj means siblings, brothers and sisters together, and saves the awkward English compound. Modern usage has grumbled about the asymmetry of -in- for decades without settling on a replacement, so fratino remains what you will read and hear everywhere.