Tenancy right under West Bengal Tenancy act

Respected Seniors, Please provide me legal suggestion in my following matter. We are holding a ground floor flat in Kolkata since 1982.Intially the rent receipt was in the name of my elder brother but after completion of agreement term on verbal request of landlord it was changed to my father's name and the tenancy was continued.In 2000 our landlord expired and converted the ownership of building to a trust and appointing her wife,sister and nephew as trustee. In 2001 the said property was gifted by main trustee - wife of landlord to her adopted daughter and son in law with an intimation to us and confirming the tenancy will continue as it is we are pay rent to new landlord.In 2002 my father expired and we requested to issue the rent receipt in name of my mother's and others I.e we 3 brothers as successor but it only issued in my mother's name.Now my elder brother has also expired and the flat is occupied by my mother, widow of my brother and his son .The total area we are occupying is around 625 sq ft super built up and paying a rent of RS 2200/- and increasing the same every 3 Years by 10%.Now the current landlord has sold the building with an intimation to us.Now the new buyer of the building is claiming a rent of RS 12500/- and asking to vacate the position.Since last so many years there is no written time bound contract and even the old landlord has not refunded our security deposit nor transferred. So please suggest me how to proceed to safeguard our tenancy Can he claim such a huge increment of rent after knowingly the rent he has purchased the building. My mother is already 87 years old will my deceased brothers wife can claim tenancy now or after my mother.