So Vyom could had a brother which he and his mom are unaware of, only his dad must have known about this which he had hid from the world as the child must be suffering from some mental issue.
he must have been obsessed with sharanya. In the flashback the one who was stabbed can be that twin, sharanya must have stabbed him and he wants to take revenge on her for his death.
i dont think shiv is that worse that he will try to molest or rape sharanya or would try to kill her. other ghost must have possesed vyom which is of his twin. vyom dad must have know all this and is keeping it as a secret as he cant expose the truth of his other son.