Inheritance

Good morning! My father-in-law (FIL) died a few years back but my mother in law is alive, which I am grateful for. Their daughter and her family have been living with them for the past 40 years, rent-free. Apparently, my FIL has left a will which is in a bank, but also an amendment, which, right now, nobody is able to locate. It sounds like when the mother dies the house will be split x:y% between the son and the daughter respectively. Additionally, the daughter will get my FIL's stock portfolio and her mother's jewelry. We don't know what else asset there is. When the house was built, my husband contributed toward its construction and several years later, paid for converting a large open area into living space to the tune of several lakh rupees. Most of the house is used by his sibling and her growing family. When the x:y% split occurs is not the fair thing to do is to deduct the amount my husband had contributed in proportion to the increase in the value of the house over the years and then do the split? Please treat my query as confidential. Thank you.