Because remember Shlok was remembering something when Malti said that Sadhna and Ragini were married to the Rajvansh family but then he couldn't remember it
Also, when they were saying bye to everyone after the puja, didn't Alekh looked at Shlok for a second time ???
Also, he is now seen playing guitar with Alekh playing the harmonica, maybe they used to do the same when they were kids