1. You need a registered Release Deed in your favour from your mother and 3 siblings to make you sole owner of the property.

2. You can appoint your mother to execute the sale deed in favour of the prospective buyer by appointing her as your attorney/agent. However it's not wise to mention in the POA that the other siblings have given away their right over the property to you and have no objection for their mother to execute the sale deed on your behalf. 

3. Since the property is situated in Mumbai, I am not aware of the stamp duty and registeration fee prevailing in the state of Maharashtra. In Karnataka, it's Rs.5000/ for the Release Deed.

4. You need Release Deed as well as POA.

Your siblings and mother have to execute registered relinquishment deed for their share in property

2) you can execute POA in favour of mother to sell the flat

3) POA should be on Rs 500 stamp paper and attested before Indian consulate  

PoA is a document authorizing someone to act on your behalf. By way of a relinquishment deed, a co-sharer sacrifices his/her share in favour of another. These two legal documents cannot be clubbed together. You need to execute and register the POA, and your mother and siblings, the release deed, separately.

You need to execute joint Relinquishment deed or joint POA for the same and get it registered. 

It is pertinent that all the other legal heirs or successors in interest to succeed to the estate of your deceased father have to execute a registered release deed in your favor relinquishing their rights in the property.

After that you an get the property revenue records transferred to your name including the records maintained by the society.

Once this process is completed, you can execute a power of attorney deed in favor of your mother or any other sibling authorising them to enter into agreement to sell or to execute a registered sale deed in favor of the prospective buyer on your behalf.

The stamp duty  for transfer of property within blood relatives in Maharashtra is Rs. 200/- 

Legal fees quoted by lawyer are reasonable 

no lawyer is going to do for Rs 2000

POA in favour of mother has to be on stamp paper of Rs 500 and attested before Indian consulate 

The charges of a lawyer towards his fee cannot be disputed by another lawyer.

If you feel that this lawyer is exorbitant at his charges, you may choose another lawyer but the fee to be fixed for such services will be done by the lawyer who you may choose for this purpose.

The fees are not much. No lawyer will do it in 1500/- sir. The legal charges in India are not so low. 

1. the flat stands in your late father's name, though the consideration is paid by you

2. so essentially your father was your benamidar and you are the true owner

3. however still your father will be considered as the owner of the flat

4. after his demise the flat devolves on his legal heirs who are his widow and children

5. mere taking an affidavit from the other legal heirs in your favour stating they have no objection if you are recognised as owner, will not be sufficient

6. the correct way is to get a release deed from the other legal heirs which has to be duly registered

7. once the flat is transferred to your name, you can then give a POA to your mother to admit your signature on the sale deed to be registered with the prospective buyer

8. the stamp duty on the release deed is Rs. 200/- or 500/- and registration fee would be Rs. 1000/- plus the scanning charges

9. the other expenses will be the lawyer's professional fees for drafting the release deed [my fees are Rs. 10k] and Rs. 15k to be paid to the registration agent + other incidental expenses if any

10. there is no need to pay under table to anybody 

11. the poa has to be on a Rs. 500/- stamp paper and has to be signed before the sub-registrar

12. you will have to make 2 separate special power of attorney - one for signing on your behalf in the release deed and another for signing on your behalf in the sale deed to be registered with the prospective buyer

13. if the buyer is financed by a bank then most probably the bank will also ask for an heirship certificate from the owner 

14. so there are expenses involved and nothing comes cheap