A. When a Hindu male died intestate (without will) him and his wife without issues (children) that property shall be devolve between class I legal heirs i.e, Children, mother and wife. In your case, she acquired her husband property.
B. However, your father being a coparcenors can exercise preferential right if the possession and ownership in joint nature over his elder's brother property under right to preemptive which confirmed by the Transfer of Property Act, 1882 under Section 44 and Section 22 of Hindu Sucession Act, 1956 and Section 2 and 3 of Partition Act, 1893 subject to offering same consideration value which have been fixed by your father elder brother's wife.
C. You can issue a legal notice to her and exercise your father preferential right. Else you can approach Civil Court and get stay order in the regard.