Legal rights of son and daughter on self owned property by parent

Dear sir, This is a case of misunderstanding which may lead to property dispute over the acquirement of rights after the sad demise of parents. In this case the property is fully owned by mother of age 80 years.Her son is retired and residing in a self owned 1 BHK flat with two sons who are still doing professional education. Her daughter is financially very strong and is a professional and happily married and has a daughter who is also going through professional education. She is staying with father and mother ever since she returned from overseas for her daughter's education. Mother who is the sole owner of the property has gifted my sister, around 1100 Sq.ft flat on the first floor which is occupied by a tenant for a very long time. The matter is in the court for a settlement with tenant, as she would like to shift in due to her professional field. The total area of property including the gift given to my sister is around 2600 sq.ft. Since mother has already gifted her the flat on the first floor which is under dispute between herself and her tenant and the case is likely to be continued for a longer time,her son wants to know legally what is his and her share in the 2600 sq.ft property after parents sad demise.