Since the flat was your father's self acquired property upon his intestate death it devolved equally among all his legal heirs consisting your mother, your brother, your sister and yourself.
Your mother is entitled to 1/4th share only ion the said property.
Your mother cannot claim the entire property for herself.
If you would wish to have your share in the property you may first try for amicable settlement, if nothing works and there is no peaceful compromise or family arrangement then you have no choice than to file a partition suit and seek separate possession of your share in it.
You dont worry that how it can be shared since it is a swelling house and very small, the court will take of it by legal procedures.