There could be multiple reasons to show this might be that when Jahnavi got pregnant with Om, Shakti and Pinky felt that if Tej has the elder son, they would have no chance of getting the business empire, and hence faked a pregnancy and brought Shivaaye from an orphanage, or maybe she gave birth to a daughter and hence exchanged it with a son in the hospital
What would be Shivaaye's reaction then, it could be very interesting.
P.S. When the show had not started, everyone was just speaking about Nakul so I thought probably Shivaaye would be the heartthrob and was very happy that Omi would be free đ, (I had not watched PKDHMM and knew nothing of Nakul before this and the promo looks made me think that Nakul would be given more footage and I have some special inclination for second leads, if I forget divinity for a moment I like Bharat more than Ram, like Bheem more than Arjun and in current serials like Pushkar more than Shravan from EDKV) but when the show started it seems that the major fan following is for Omi đ đ , so I have decided to focus on Shivaaye at least he is free on this forum