Possessing property without registration

A close friend of mine is in possession of a residential property. His father purchased the said property more than thirty years back paying then agreed amount (total and full) and getting Agreement-cum-Attorney registered. Later his father got the house assessed, and started paying House/ Sewer/ Water Tax, Electricity Bill etc and also got the extension/ modification map approved by the concerned ULB/ Development Authority. His mother passed away before his father, who himself passed away nearly fifteen years back. The seller and attorney (His father’s friend) passed away before his father. However, neither his father nor he could get the property registered on their name so far. After demise of his father my friend has continued paying taxes and bills though his sister is also residing in the property. He is interested to file a case against his sister to get the property registered on his name. However, he fears that court would dismiss the case as the property is not even registered on his father’s name. You are requested to give you valuable legal advice on the above and the following critical issues: 1. What and how can he get the property registered on his name (assuming that his sister would not create much of the problem, and may agree with social motivation)? 2. Is registration necessary in case he is interested to sell it off? 3. What other legal formalities he is supposed to do? 4. It is clear that tracing the seller’s legal heir is not at all possible. Which laws are applicable in the light of possession of the property, payment of taxes and bills etc?