Re: Mother tongue.
Its mother’s language as per my observation. My Sindhi speaking uncle married to Urdu speaking family. They lived in different cities of Sindh including Karachi, Hyderabad, SanghaR due to my uncle’s job. My cousins speak urdu at home and communicate with us in Sindhi, which is quite good compared to other such families, considering they lived in areas,where Sindhi was spoken. Had they lived in areas, where Sindhi was not spoken, then they could not have communicated with us in Sindhi