It can be because she is a recent widow and the child might be her husband's. But then it is no reason for her to be killed for that.
So the conclusion has to be it was after her husband was dead she got pregnant with another man's child. Why no one seems to be talking about this?
It is a kind of situation where two people are involved and someone who is ready to kill his own daughter for this kind of "disgrace" will be intent on killing the person who was also to party to it.
There is no reason for everyone to assume that the father of the child is not ready to take the responsibility without knowing more. But it seems to be a child sired by Saanvi alone.