Negligible rent

Hi, I am writing to you on behalf of my senior citizen mother who has an ancestral property in Darjeeling, WB in the heart of the town wherein there are 6 units put out on rent. As they date back around 50 years plus there was no legal agreement done nor any rent receipts issued. Each unit pays 50/- to 100/- pm. Where as the market rate is around 1500/- to 2000/- pm. 2 out of these units are locked and the tenants have moved away to their own new houses but still refuse to let go coz of the cheap rent. As such it's not maintained at all.And on the other units, their next generation has taken over. Considering that these rents are her only source of income ( besides my financial support)....it's too meagre for existence itself. In such scenario: 1) Can my mother increase the rent which has not been done for the past 20-30 years to the market value?? 2) If there is no mutual understanding with the tenant, what are her legal options/ rights to either get the premises vacated or getting the tenant into new rent agreement in documents. Thanking You in advance ML