Father's ancestral property divided before birth. How can i claim it?

When it comes to ancestral property, one can execute a will only with respect to his share in the ancestral property. In your case, your father had executed a will with respect to his share in 2008 and that is allowed within the ambit of Hindu Succession Act and therefore you and your mother have right over your father's share in the ancestral property.

Your father's brother cannot claim your father's share as Section 6 of the Hindu Succession Act clearly says that Where a Hindu dies after the commencement of the Hindu Succession (Amendment) Act, 2005, his interest in the property of a Joint Hindu family governed by the Mitakshara law, shall devolve by testamentary or intestate succession, as the case may be, under this Act and not by survivorship, and the coparcenary property shall be deemed to have been divided as if a partition had taken place.

Hence, your father's brothers claim shall not succeed in the Court of law.

You should file for a probate case for the execution of the Will if the said Will is in dispute (please note if not disputed, the probate of the Will is not mandatory in Delhi). Once you are able to prove that the Ancestral property was partitioned among your father and his brothers then you can vindicate your stand.