Are the heirs entitled to claim their deceased mother's share of her previously deceased son's estate, and is retracting the waiver of this share considered a sin?

The death of a mother before the distribution of her deceased son's inheritance does not nullify her right to inherit. She is entitled to one-sixth of what her son left behind. This portion is added to her own estate and then distributed among her heirs (her sons and daughters). As for renouncing the right of inheritance in favor of the nephews and then retracting it, there is no sin in that, because a gift is not binding until it is taken possession of.