You implied that Pashto and Urdu are sister languages...that Sindhi and Urdu are sister languages - They are not ! Barring the odd word out there is little in common between them ! What I said was that Urdu was our lingua franca which is to say 'our common tongue'; which to put it ever so simply means that almost every Pakistani knows 2 languages - One his Mother Tongue and the other is Urdu ! And we've know this for so..so many years before the Partition !
Bengali being the majority of Pakistan's language doesn't count for squat when it isn't a non-partisan language or understood by us ! Urdu was understood by people in East-Pakistan just as Urdu was understood by the People in the far-off coastal areas of Makran and the lands up North in GB ! How else do you galvanize a nation into One without giving them a language that all of them could claim ownership over ? How can a language explicitly associated with one particular ethnic group...just one be made the National Language of the whole country ? They tried that in KPK with Pashto and guess what the Hindkowans spoke up against it ! They tried that with Sindhi in Sindh and guess what the other ethnicities resented that ! They haven't tried that with Punjabi yet but I can bet my arse the Sereikis, the Punjabi Pathans, the Punjabi Kashmiris and so many other ethnicities that inhabit this province would speak up against this !