Family Land dispute

Hi Sir, I am working in bangalore as engineer and my mom & dad is living in bihar, which is our ancestral place.My uncle has built an illegal building on our land which belongs to my dad & uncle.The building is built in such way that it acquires around 70 % of the land.The land is still not divided between the 2 sons and when it was built we were told that we can given the half building expenses when I would start earning.My father is farmer and initially we were very poor.When I tried paying the expenses he is not taking and telling us leave the ancestral property. Since my uncle was a rich guy hence we cannot do any thing at that time.Now we want to make our own house and want the half of the land.We are not being allowed to so by different means such as harassing my parents with local goons.So I have decided to take legal action on this to protect my parents and my ancestral land. So let me know the way forward to protect my parents and get my land back.Once they have abused my mom so we have filled a case in police.So let me know the best legal action in this regard.