A. In your case, I can presume that two brother intentionally sold the land to your father by hiding her sister.
B. Basically, the said two brothers were cheated to you by suppressing material facts towards their property. Hence, they are liable to resolve this case without receiving any cost from your end. I hope this indemnity clause might have seen in your Sale Deed also.
C. Secondly, the determination of share of sister of the two brother yet be to be determined by the Court as to whether the suit property was ancestral property or not? if it was ancestral property, as per the latest Supreme Court decision, no equal share will conferred to her if father died before 2005. And you will also be protected under the bonafide purchaser subject to paper publication or discretionary of court.
D. Thirdly, the question of law of limitation which means why she was silent till last year even the Sale was happened in the year 1994. However, the court has reserved the right to condone the delay of time. However, presumption is Court never entertained this type of suits based on delay.
E. Finally, you can challenge the case and you can alienate(Sale, Gift, Mortgage and Exchange) the property subject to absence of Stay order and the Court final verdict.