Is a verbal will, which was not written down, in which the grandfather and grandmother relinquished their share in an apartment to their deceased daughter's son and daughter, considered binding on their heirs after their death, and is he who does not abide by it sinful?

For your grandparents to waive their share to you, it requires proof through evidence or the acknowledgment of the heirs. If you took possession of the share before their death and disposed of it, then the gift (hiba) is complete, and your maternal uncles and aunts have no right to it. However, if the waiver was merely a statement and you did not take possession of the share until they passed away, then the gift is not complete according to the majority of jurists who stipulate possession for the completion of a gift. If the donor or the donee dies before possession, the gift becomes void. This waiver is not considered a will (wasiyya), because a will is the transfer of ownership after death. If they did not bequeath their share to you, then it belongs to their heirs.