Yea I agree that they should tell her and that sounds like a perfect idea to tell her just that she's their daughter and she left for so and so reason but bringing her up will hurt everyone so please don't.
But to do even that is going to take them time. For a family that is as close as them, losing one person like that was probably traumatic, and since it suddenly came out in the open, it's gonna take them a bit to collect themselves before they can talk about it. Going by precap convo between Shappu, I'm sure Appu finds out about it tomorrow.
But the thing is, everyone knows how stubborn and honestly immature/childish she can be, so they can also be apprehensive about telling her something like this. Doesn't make it right still and they should still try to explain it all to her but because she has a right to know not because of her curiosity to know everything or her stubborn behavior.