What is the relationship between these languages? As almost all the classical Punjabi literature that is thought to be Punjabi comes from Seraiki (baba Bulleh shah, Farid Ganj Shakkar, Waris Shah and Sultan bahu).
Re: Punjabi versus Seraiki
Baba Bulleh Shah, Waris Shah's kalam is in Punjabi. Shah Hussain and Baba Fareed did poetry in Saraiki. Shah Hussain's majority kalam is also in Punjabi.
As far as link between two languages is concerned they got vast common vocabulary and Saraiki is also got influence of Sindhi too