Sheikh Husayn al-Jabûrî, professor at Umm al-Qurâ University
One of the reasons someone inherits is that he is alive when the owner of the estate dies. If the brothers and sisters were alive when the owner of the estate died, then their children deserve their fathers’ or mothers’ shares if they have yet to be distributed.
The reason for this is that these children would be the inheritors of their parents and accordingly they have the right to receive their share.
However, if any of the estate owner's sons and daughters die before he does, then their children have no right to receive any share in the estate after the death of the owner of the estate.
Some scholars have said: In such a case, the father may prepare a will in which he may assign the share of his deceased son or daughter to their inheritors. This is only applied in the Egyptian courts.
And Allah knows best.
Source: Islam Today