So, after seeing the power-packed episode of Naagin 5, we came to a conclusion that Balwant was right regarding Maarkhant. So, let us come straight to the topic, we may have doubts like 'If Markhant is Jai's mom, then how could he travel in the flight with his mother?'
As much as I seen, I have two conspiracies regarding how they could be siblings.
1) Hriday was Maarkhant’s son in Satyug and Veer was Maarkhant’s son in Kalyug. Jai might be born to a normal naagin in this birth and travelled in Blue Star Flight which Balwant crashed. But Maarkhant is living since 10,000 years… So, when they connect the dots, Veer and Jai were brothers.
2) Knowing that Maarkhant travelling in flight, Balwant crashed the plane in order to kill her.
Second one is complicated and somewhat illogical, so First one might be the actual theory regarding the relation between Veer and Jai. But second one gives a justification why he wants to crash the flight
Just my views, share your opinion