As per the given query, the appeal is still pending in the Higher Court.
It is not allowed to sell the property that is the subject matter of the litigation.
According to section 52 of Transfer of Property Act, sale/Transfer of a property which is the subject matter of the litigation is not void but voidable at the option of the transferee.
However one cannot do so, so that no interest in the property is lost to the prejudice of other party to the suit.The sale that is made by the cousins of your deceased wife will be hit by the Doctrine of Lis Pendens.
It means that the subject matter of a litigation should not be transferred to a third party during the pendency of the litigation.
In case of transfer of such immovable property, the transferee becomes bound by the result of the suit.
You may make an application to the court to grant a stay on such a sale which has happened. The court has powers to restored the property which has been already sold.
It is not illegal to sell the property which is the subject matter of the litigation but the transferee will be bound by the decision of the court for whatever is decided with respect to the property in question.
Reference: Transfer of Property Act,
Section 52- Transfer of property pending suit relating thereto.ÑDuring the [pendency] in any Court having authority [[within the limits of India excluding the State of Jammu and Kashmir] or established beyond such limits] by [the Central Government] of [any] suit or proceedings which is not collusive and in which any right to immoveable property is directly and specifically in question, the property cannot be transferred or otherwise dealt with by any party to the suit or proceeding so as to affect the rights of any other party thereto under any decree or order which may be made therein, except under the authority of the Court and on such terms as it may impose.