How deal with my in laws

My father in law have one son (my husband) and a youger sister who married 8 years back n have a girl child with 5 years above. My father in law got retired with huge amount and now he is getting 1L pension where at coming months pension will increase also. My husband is small private employee n earns 40 thousand. We never depend on parents. We too have lot of problems likes from last 10 years to child to us. We are hoping my father in law to save money for our family use in coming days because they are also older n for us also age is coming. But my husband's sister after marriage also always be in my father in laws house. Now she got a small government job n permenantly sitting in my father in laws house. She always interfers in our family matter n wrong directs father in law, n directs all money to her side. Eventhough my father in law is a good government employee, till now he didnt save any money. If we ask where you spent money, my father in law with his daughter are fighting with us n saying we have no right to ask, they dont want son, n saying daughter have rights to be with them n eat they money etc., which are very hard to listen. My husband is suffering internally, what to do with this issue. For society, son is 1st responsible for parents n should see them at old age. But here son have small job n asking parents to have money on there name for there further use. As a middle class family, what my in laws are getting as pension is very much high. So only we are asking them to save it on there name but my father in law saying we dont have right to speak about there finance. Please anyone of you can give good suggestion for below my queries: 1. How can we know the expenditure of the house of my father in law eventhough we are living in another city as of job purpose? We dont want any rupee from them except the expenses. 2. We think knowing the house expenses is the right thing to know where all money is going on and make them save it. We already asked them with normal talk but they denied it. Is this our thinking is right or Is there any other way to deal this? 3. How we can approach this matter legally? We opted legally because for society or relatives it seems that my father in law is giving everything to us. Already some of our relatives are advicing that seeing parents would be our responsibility, which we too agree. But no one is aware that how he is wasting money, saying us harsh words that they will not be with us at old age, which seems like we have taken there hard earnings n telling them to go out. Thank you for considering my queries.