1. Your mother after intimating her sister of selling the property can initiate the sale process of the land when her sister is still a tenant.
2. If the two sisters have maintained cordial relationship, then the sister who is a tenant, may likely to vacate the premises. However, in the absence of Lease or Rental agreement, your mother's sister may not pursue legal action.
3. Your Mother has to first send a legal notice to her sister for vacating the place and if the problem still persists, your mother has to approach the competent Court for evicting her sister from the premises.
4. Yes, it's possible.