Forced to get into another rent agreement by family member

I am a shopkeeper.Last November 2017 I gave advance by cheque for a shop to Mr.x . Without any legal documentation,and he gave me the key of the room and I took possession.He told me to open my shop.After some days we both agreed to do an agreement later.I have also paid him 1 month rent by cash.After opening of my shop within 4 weeks all on a sudden he passed away due to heart attack.Later I came to know that Property was in his mother's name.But the building he constructed over it is in his name .As per the municipality law the holding number is also his name.the siblings of Late Mr.X are claiming that the property is all theirs including Mr. X they are 7 brothers and sisters.4 Brothers and 3 sisters.2 brothers are died and another 2 brothers and 3 sisters are alive now.Before 4 months 6 members of their family did a rent agreement with me jointly.Now the son of Late Mr. X force me to do another agreement with him alone and show me a will paper of his Grandmother by his name.It is in a plain paper before 18 yrs back.With the signature of 2 witnesses.Now his Grandmother and the 2 witnesses are dead 10 yrs before.I am paying my rent from last 6 monts to the previous party who did agreement with me.Before 7 days they suit a file in sessions court.Please suggest me what to do ?