oh i'm so sorry to hear that, i feel for the whole family, as for your 5 year old nephew, i wouldn't take him to the grave, i know my son who is also 5 would understand that being with Allah is a good thing but not understand why the grave is a better place,i could imagine my son asking questions such as is she not cold there, is she not lonely, does she not get scared in the dark, i personally think you should not take him to the grave yet... I just pray that Allah gives you all sabr and gets you through this difficult time, Ameen
I agree. I don't think it would give him any comfort to think his little sister is inside that grave... better keep telling him she's in jannah and one day all of us have to go there. The best thing would be to distract him and keep him occupied with other things as much as possible and 'behna' shouldn't be mentioned in front of him at all.