1. What do you mean by her husband's matrimonial property? Do you mean that he claimed that the money was provided by his parents? After the enactment of Benami Transactions (Prohibition) Act, 1988, the property registered in one's name can not be claimed by other. In the instant case, the husband shall have to establish with irrefutable evidence that he had provided the fund to her wife to buy the said property which will be impossible at this stage.
2. I do not know whether he will be able to prove or not. If the fund was provided by your mother then how will he prove such funding?
3.Land's value will be determined by the value at which it has been registered to be sold. Now a days, the Registry Office charge stamp duty considering the market value of the property saloe deed for which is registered.
4. Under value or over value of the property is not the concern of the husband. It has been sold at a value and if he claims any share it should be from the sale proceeds receive by his wife, not on his assumed figure.
5. If Court allows him 50% share it will be on the sale value actually received by his wife and not on his assumed value.
6. Your sister shall have to state that the money was arranged by her parents and she can also arrange to show accounts to prove that she had received the fund from her mother (when she had come to India) and/or other relatives as gift.
7. Your sister can engage a detective agency to trace out his properties or can ask the Court to direct him to provide property details and also the I.T Authorities to submit his last 5 years I.Tax return where he is expected to show purchase of properties and bank balance.