Title and possession of proprty

Sir/ madam my father had brought 7 gunta(3+4 gunta) land in a village in maharashtra in the year 2004 and 2013 by two seperate sale deeds from two real sisters. same has been mutated and entered in 7/12 record.but in year 2016 three brothers from same village came up with other sale deed from third person about purchase of house proprty and land and forcefully started construction of building in my fathers owned land.(That house was used for road side tea stall from years adjoining main state highway.) they claimed that they posses property from seperate Gat No/ Survey No. and not part of our land and encroched in our land.This said land is Agri land and these enchrochers names neither that third person's names not present in old/ lateset 7/12. Since this proprty is out from Gaothan area hence there is no Proprty card for said land+ building.they claimed their possession on Grampanchyat revenue assessement. which clearly says that assessement is meant only for tax collection and can not be treated as ownership right.They enclosed this assessement certificate as title/owner during sale deed.While we approcahed lower court for injuction but court dismissed our application on ground that gram panchyat has given permission for construction on civil engineers plan.and defendents are making construction on their own land.The third person who sold this proprty to these three brother (defendents)does not have any proprty share in that Gat No..however during consolidation his mother gave up all rights from that Gat No and taken other part of land ceasing all rights in dispute land gat no. my question is 1. which documents are clear evidence of title of property? 2 can some body execute sale deed of agri land and farm house on assesement certificate of gram panchyat? 3.If land is agri land then which documents states ownership / title?. 4.which documents are mandatory for sale deed of agri land + farm house. 5.It is clear that seller and defendents are unlawful possesser and tresspasser bcoz they dont have 7/12 and property card but ther have taken benefit of improper direction mentioned in our sale deed. now plz suggess how to prove my fathers title and challange there unauthorised sale deedi in house of law/court? presently case is in lower court at initial stage... please guide me . thanking you