Son and daughter rights in a property

Hello, We are 2 sisters and one brother. I am eldest sister. After my marriage, my parents bought a land in 2001 after selling their previous self bought property in Delhi. Earlier and this property were both in my mother's name while she is a house wife. They took bank loan to build a house on that. My brother (middle child) was made a coborrower in that. My parents are staying in this house with my brother and my young sister (married and citizen of Australia now) is settled abroad. I am worried that my brother will coerce my parents to make a deed in his favour so that later on we sisters do not get any share in that property. It is possible that he has already done so. My brother is very money minded and he can go to any level to get the property (even if he has already bought some other properties already). My questions: 1. Does my young sister have any rights if my mother makes any deed or will in my brother's favor? 2. Does my young sister have any rights if my mother dies without making any will or deed and my father would be alive 3. If my mother has already made any such deed or will, can she change it now? I am sure if I talk to her, I will be able to convince her. 4. I have 2 daughters and one of them is 18+ now. Does she have any right in any case? 5. If my mother dies before my father without making a will, and my father makes a will in my brother's favor, will it be valid? Regards, Sudha Sharma.