Either it'll be something like a marriage or something. Or it'll be something related to Malhar's friends and childhood. But I feel this is also less likely as everyone is convinced that Kalyani taught him everything before she brought him.
But the thing is that even in today's episode, everyone was stumped how Malhar was able to answer all the questions correctly.
i think childgood ya phir vo ancestral property jisko leke hum discuss karthe he vahi hogi
like identify their pustheni ghar lya their property ( which kallu wont be knowing) ya phir uska koi old friends ke ghar
ek aur shadi chance less koi bts na aayi na