In historical linguistics, sister languages are languages that are descended from a common ancestral language. Every language in a language family that descends from the same language as the others is a sister to them.
A commonly given example is of Urdu and Hindi (the standardized registers of the Hindustani language), which are mutually intelligible with each other. Also, the Romance languages, each of which is a continuation of Vulgar Latin. Italian and French (both Romance languages) have about 89% lexical overlap, meaning 89 percent of words share the same characteristics and root origins. Similarly, Spanish and Portuguese also have about 89% lexical overlap. Spanish and Romanian overlap less, about 67%, because while Spanish and Portuguese have undergone Arabic influence, Romanian has undergone many different influences over the years, particularly from the Slavic languages and Greek. Along with a large amount of shared vocabulary, the Romance languages share numerous features of morphology and syntax because they are all continuations of their common ancestor, Latin.