As per the horoscopes, Ranvijay has no son, but Shekar has. The manners how they have been brought up and the cultural environments surroundings them, I think Ranvijay believes in the prediction by the horoscopes, though he pretended not. So, He laid out his best ploys:
Made them (Laali and Shekar) drunk, on wedding night. After Laali and Shekhar already slept together (consummated) in drunkard state, Ranvijay shifted Shekar and he slept besides Laali and created all the dramas of him together with Laali. He shoots two birds with one stone. The boy child born will be his to take the reward as father of heir (who does DNA test in the village 😆) and Shekhar will go crazy when he learns that Laali slept with Ranvijay (his mental conditions will sway so badly that he may be sent to mental asylum) and would be out of his (Ranvijay's) ways.
That's what I am guessing. What's say friends?