In KMH-1, they never revealed this truth. So I guess this was killing Kekta because she missed another opportunity of creating drama in the lives on Arjun-Arohi, so she was hell bent on revealing this truth this time around.
In KMH-2, from the coldness of Arjun's current parents it is definitely clear that Arjun is their adopted son. Their concern for Mikail over Arjun gave it completely away. With Arjun, the parents share a more cordial relationship. However, from Arjun's sad expression I think he knows he is adopted.
Good prediction and if you don't mind I'll like to add to the prediction. Since I have watched a lot of Balaji serials, I think Arjun is the son of Rudra's partner and I think Rudra Sighania killed both Arjun's mom and dad. Arjun will find this truth out sooner or later with the help of Arohi and then eventually will go against the Singhanias and will shake hands with Alluwalia and put the Singhania's behind bars. Eventually, for helping the police, Arjun will not be in prison for a long time (he even might not go to prison) Arjun-Arohi will live happily ever after. If this is not the case, then I really wanna know how a police girl unite with a gangster guy...!!!...