Dear Alexis,
No, your older brother can no longer contest and have the transfer of ownership to you nullified.
When your sister died, ownership of her properties immediately passed to your parents as her legal heirs. With this, your parents had the right to dispose of the properties however they wished. Even when they executed Waiver of Rights over the properties, these were effectively intended as donation to you.
In the case of Felix Cruz v Robin Cruz, G.R. No. 269039-41 (19 February 2026), the Supreme Court reiterated that under Article 749 of the Civil Code, a valid donation of real property requires: (1) a decrease in the donor’s property; (2) a corresponding increase in the donee’s property; (3) intent to donate; (4) execution of the donation in a public document; and (5) acceptance by the donee. If the acceptance is in a separate document, the donor must be informed.