1) As advised above, it is presumed the first wife was legally wedded wife, and the second marriage after the death of first wife.
2) The property belongs to the deceased first wife not your deceased father.
3) The law doesnt distinguish the property is whether ancestral or self-acquired in the instant case.
4) As per section 15 of the Hindu Succession Act, 1956, 15(2)(c), If a female Hindu leaves any self-acquired property, in the absence of husband and any son or daughter of the deceased (including the children of any pre-deceased son or daughter), the said property would devolve not upon heirs as mentioned in sub-section (1) in the chronology, but the heirs in category (b) + (c) would inherit simultaneously. If she has no heirs in category (c), then heirs in category (b) + (d) would inherit simultaneously.
5) Her mother-in-law and mother should equally inherit her self-acquired property. Her own brother and sister should equally inherit along with her brother-in-law and sister-in-law.
6) The property of hindu married widow issueless died intestate devolves upon all the legal heirs.
7) You will stand along with other legal heirs as per amended succession, not simply you are the only legal heir.
8) Hence as per the above stated legal position, consult local property law expert advocate to clarify the position after ascertaining full facts of the case.