Building bylaws relating to floors differ from one state to another.
In Tamil Nadu, according to the THE TAMIL NADU APARTMENT OWNERSHIP ACT, 1994, Section 3 (e)-
“building” means a building containing five or more apartments or three ormore floors and comprising a part of a propertyThis law covers the property in question, as it has four floors and thus meets the set criterion that defines a building under this act. Under sections 11 (1) and (2) of this act, the floor plan has to be registered compulsorily under the Registration Act, 1908 (Central Act XVI of 1908) . Since your sister has not adhered to these norms, it is a direct contravention of the said law and you can accordingly question her ownership over the said property.
Furthermore, you may consider filing a case citing that the transfer of property was not under free consent , you can cite that your deceased mother's emotional appeals had "coerced" you to transfer the property at nominal rates. Sections 14 and 16 of the Indian contract act define "free consent" and "undue influence respectively".
Under section 16 (1) of the Indian contract act- "A contract is said to be induced by ‘undue influence’ where the relations subsisting between the parties are such that one of the parties is in a position to dominate the will of the other and uses that position to obtain an unfair advantage over the other"
Under Section 16 (2) of the Indian Contract Act- A person is deemed to be in a position to dominate the will of another—(a) where he holds a real or apparent authority over the other, or where he stands in a fiduciary relation to the other; or(b) where he makes a contract with a person whose mental capacity is temporarily or permanently affected by reason of age, illness, or mental or bodily distress.
Therefore, you can use the provisions of Indian Contract Act to your advantage as well by citing that your mother's compulsion was a case of "undue influence". File a suit as soon as possible, since there is a limitation period of 20 years for immovable property disputes, after which you cannot approach the court.