Is it permissible to give the nephews a pledge not to demand their usurped legal inheritance, in exchange for maintaining kinship ties, knowing that the uncles and aunts are aware that the relationship will then be based on hypocrisy and lies?

Your relinquishment of your right to your nephews is permissible and does not constitute an affirmation of falsehood. However, you do not have the right to waive the rights of your siblings. Courtesy and diplomacy, when intended for reconciliation and repelling evil with good, are not considered hypocrisy. The statement, "Whoever walks with his opponent while the opponent is unjust to him, then he is sinful until he returns to the truth," cautions the oppressor against bringing the matter before a ruler when he knows that the truth is not with him, because the ruler's judgment does not make the unlawful lawful; rather, the reckoning remains for the Day of Judgment.