Cancellation of Sale Agreement

Let's name a person as A. Mr. A has 2 wifes. W1 and W2. W1 has one daughter (W1.D1) W2 has 3 sons and 1 daughter (W2. S1)(W2.S2)(W2.S3) and (W2.D4). Daughter of 1st Wife (W1.D1) has 4 children ( 2 sons and 2 Daughters) Mr. A has 4 properties. The legal heirs of 2nd wife has went into a partition agreement on 1963 without including the legal heirs of the first wife and sold one property on 1974. They have again sold another property on 2003. Now the 4 children ( 2 sons and 2 Daughters) of 1st Wife (W1.D1) has filed a suit on 2004 as they came to know about the sale only in 2004. The suit claims that Daughter of 1st Wife (W1.D1) is entitled for 1/5 of the share in all the properties. My question is whether the suit is valid? And whether Daughter of 1st Wife (W1.D1) is entitled for 1/5th of the share? Plaintiffs are Children of 1st wife's daughter. Defendants are 2nd wife's sons and daughter. Defendants replied for the petition that the property has been maintained by the sons and daughter of the second wife. Hence the 1st wife has no valid right to the property. Is this legally a valid point? Coming to the main query. The plaintiffs (Children of 1st wifes daughter) has signed a registered sale agreement on 2005 with a third party as per the instructions of the plaintiffs advocate that they are agreeing upon a sale agreement for 1/5 of the share of the suit property. Sale agreement mentioning the case number states that the plaintiffs are agreeing to sell the property for Rs. 129000 to the third party once the decree is passed in our favor of the plaintiff and the agreement also states that an advance of Rs. 100000 was paid to the plaintiffs. (For easy understanding i have used the word plaintiffs. In agreement no plaintiff word is included). Later we came to know recently that the third party whome the sale agreement was in favour is Plaintiff's Advocate's Son. Now we are cornered. Even if the decree is passed in favour of the plaintiff, the property should be sold as per registered agreement or is there any way to cancel the agreement? My Questions: 1. Can we claim right over the sold property and take the appropriate (more) share from the remaining unsold property? Or only 1/5 share from the unsold property is eligible? 2. Is our claim over the property is correct and legal? Will the decree be favour on Plaintiffs? 3. If the decree is passed in favour of plaintiffs, can the registered sale agreement be cancelled after the decree?. Out of 4 parties from the plaintiff who has signed the registered sale agreement, 2 parties have died. Please guide us in this regard. Thank you so much for your time and guidance.