Sindhi is an ancient language, over seventy percent of the Sindhi words are Sanskrit. The fact that Sindhi is mostly written in the Arabic script, gives some people the impression that it is a Persio-Arabic tongue. Professor E. Trumpp in his monumental `Sindhi Alphabet and Grammar' ) writes: "Sindhi is a pure Sanskritical language, more free from foreign elements than any of the North Indian vernaculars."
Sindhi language is said to have developed from the Virachada dialect, which originates from the Prakrit language. This was during the 1500 BC and it was categorized as an Indo-Aryan linguistic. Find out more from http://www.alsintl.com/resources/languages/Sindhi/.