On parents demise you have one third share in property 

2) issue legal notice to brother fir division of property by metes and bounds 

3) if he refuses file suit for partition for division of property by metes and bounds 

You need to properly partition the property with a registered deed or else file a case ik court for proper directions

It is as of now a common property. You can break the lock and enter into the room.

Or you can make a police complaint for not allowing you to use the room forcibly.

Finally you can file a partition suit in civil court.

See you can file a partition suit before the civil court you , your brother and your sister shall have equal share in property if same cannot be divided by meats and bounds court shall order auction of same and divide proceeds.

You can break the lock and have forceful possession of second floor. retaliate.

All three have 1/3rd share each in building. Partition suit will file to bring finality of right in property and possession.

1. For proper separation of property you have to file a suit for partition against your brother and sister as defendants of the suit. 

2. Till decision of suit you can pray for interim relief and stay for opening of room on your floor through Temporary injunction against your brother till decision of suit.

3. Or you can forcefully break the lock and open the room for your use as your brother already have possession of complete ground floor. 

The property left behind by your deceased father or mother shall devolve equally on all the three legal heirs.

Your brother cannot take away the entire property or major share without the consent or NOC of other shareholders.

Your sister is also entitled to an equal share in the property hence she also can file a partition suit seeking partition and separate possession of her legitimate share in the property.

You can either stand on the side of your sister or can be on the opposite side and can claim your equal share too, the court will decide based on the merits accordingly.

If it is the property of your parents, your brother Can't diprive you from your legal right, you tell me that you have a equal share, if your father didn't made any will then you can file a suit before the appropriate court and can claim your right in the property. 

Since the property is undivided you and your other sibling can file a suit for partition to cull out the separate possession of your share in the property through partition by metes and bounds.