I found an article PEOPLES AND LANGUAGES IN PRE-ISLAMIC INDUS VALLEY by Dr. Tariq Rahman, Fulbright Visiting Fellow (U of Texas).
It explains in detail where the similarities came from between Sindhi, Punjabi and Saraiki.
Sindhu-Sauvira region existed back in 130 A.D and comprised of modern day Sind and Lower Punjab.
The countries of Sindhu and Sauvira are mentioned in the Mahabharata and have been taken to be roughly the present province of Sind and lower (i.e. Siraiki) Punjab. Some scholars, however, consider them âneighbouring countries of the Punjabâ with Sindhu on the west and Sauvira on the east of the Indus.76 A.H.Dani, however, locates Sindhu roughly in the province of Sind and Sauvira, in his opinion, âdefinitely lay to the east of the river Indus much higher up.â This leads to interesting linguistic hypotheses which are best given in Daniâs own words as follows:
If we accept this suggestion, it is not difficult to understand why the Sindhi language is confined to the lower Indus while Saraiki is now spoken in much the same area where Sauvira is located by Alberuni. With this understanding of the Saraiki-speaking area, we can now say that the very name Saraiki is probably a corruption of the original term Sauviraki.
According to Grierson the mother of Sindhi was Vrachda. It was the spoken language, or Apabhramsa, 'of the country round the lower Indus. It was also the mother of what Grierson calls Lahnda and what are now known as Siraiki and Hind Ko.
It is possible then, as A.L.Turner opines, that Sindhi must have separated from the mass of related languages sometime between 250 B.C. and the first century A.D. John Bordie, using linguistic evidence of loss of certain words per thousand years, suggests that Sindhi and Punjabi separated between A.D.750 to 1400 and that the implosives of the Sindhi language 'came into existence prior to A.D. 1400 and subsequent to the separation of Sindhi from the mass of related languages. Since Siraiki too has implosive sounds, it too may have become a separate language around this period. But Siraiki shares its vocabulary, or at least a major part of the core vocabulary, with Punjabi so that the present writer is unsure whether Siraiki is a sister of Punjabi which picked up some features of the Sindhi sound-system (phonology) or a sister of Sindhi which picked up Punjabi words as Grierson suggests.
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