In historical linguistics, sister languages are cognate languages; that is, languages that descend from a common ancestral language, the so-called proto-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 the Romance languages, each one of which is a continuation of spoken Latin. Italian and French have about 89% lexical overlap, meaning the words share 89 percent of the same characteristics and root origins. Italian and French have a huge number of similar words. Similarly, Spanish and Portuguese have about 89% lexical overlap, so many words are shared or similar between those two languages (see also cognates). Spanish and Romanian's overlap is lower, at about 67%. Spanish and Portuguese have undergone Arabic influence and 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.
When considered a separate language, the Scots language is considered to be a sister language of English, as they are both descended from the common ancestor Old English (via Early Middle English). (Some people consider Scots to be a register or style of Scottish English.) The phonological development of the two languages is divergent, with different loanwords entering each language from sources such as Norse, Latin and French. Political and cultural events have largely dictated the decline of broad Scots as a standard variety in the modern period, and Scots is currently confined to largely spoken use and unofficial functions.