Is it permissible to retract part of a sum of money that was previously relinquished, especially since the relinquishment occurred without consent?

The gift from your elder sister to your uncle, a portion of his debt, falls under the category of gifting a debt to a non-debtor. The validity of such a gift is a matter of scholarly disagreement. According to the view that deems the gift valid, it is not binding until possession is taken. Since you have not taken possession of the gift, your sister has the right to revoke or modify it. Furthermore, a sister is not obligated to be equitable in her gifts among her brothers and sisters. Equity is primarily obligatory among children, although it is preferable to be equitable among relatives to foster harmony and affection.