What share will I get in property

I have two elder brothers and elder sister. my father and mother both died before my marriage and they did not write any will.We have two buildings and one in mothers name and other in fathers name. I do not have original property documents or duplicate copies. (sub registrar office also don't have copies as the were burnt. my brothers and sister resides at the same property and gets rental income from the property and they are enjoying without giving my share.my elder brother got one building on his name using mutation endorsement 17 years ago and he got property tax and electricity bill on his name while my father is alive.(how long this is valid?) they are threatening me and not giving my share. how can I get my share and what documents needed.they are also trying to prepare will using forgery. If I proceed legally they will abandon me and how the court will decide which portion I will get in the building as it is three storeyed building with 9 portions.please advice.