While Amitabh Bachchan and Waheeda a Rehman play Abhishek Bachchan's grandparents in the film, Rishi Kapoor plays his surrogate father. Incidentally Waheeda Rahman was Amitabh's real mother in Coolie while being Rishi's surrogate mother. In fact after playing the leading lady opposite Big B in films like Adalat, Mahaan and Kabhie Kabhie, she played his mother in films like Coolie and Trishul.
After acting with Rishi Kapoor in Coolie, she came together with him in only one project - Chandni. However, in the film the two were not related as she played mother to Vinod Khanna, film's second lead.
Now in Dilli 6, one waits to see how fireworks explode on screen once the trio of the 70s/80s come together with the likes of Abhishek Bachchan and Sonam Kapoor.