My prediction of the future track is that Arnav's dad and mom had their child as Anjali .. after she was born, Mrs Raizada may not have been able to have any more children and since they wanted a boy to carry on the family name, Arnav's Dadi would have brought in Garima to help. She would have had an affair with Arnav's dad and conceived Arnav.
Once the child was born, Garima left and the child remained with the Raizada family and was brought up by Anjali's mom.
Since they were Malik's at that time, Garima has not yet been abel to connect the dots to realise that ASR is actually Arnav Malik.
Now on seeing Dadi she has made the connection and so has Dadi. Dadi would have been under the impression that Arnav and Khushi are half brother and sister (with Garima as the mom) till she heard from Arnav that Khushi is Garima's adopted daughter. This technically makes Payal and Arnav related.
Just my prediction guys :)