What is the ruling of Islamic law regarding an aunt relinquishing her property to one of her sister's children before her death, in order to disinherit her brother, knowing that the recipient has disposed of some of this property before the aunt's death?

If your aunt's relinquishment of her property occurred during her death illness, then you are only entitled to one-third of it, and the remainder goes to her heirs. If the relinquishment was conditional upon her death, then it is a will, and you are only entitled to one-third of it. However, if the relinquishment occurred while she was in good health, then the relinquishment is valid, and your possession of the money is valid, because a person is permitted to donate all of their wealth while in good health. It is advisable to give some money to your uncle to maintain kinship ties and earn its reward.