Property dispute

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I have an parental flat in Kolkata. Legal Heirs - four sisters.

One sister wants to buy the flat where 2 other sisters have agreed. One sister is not agreeing.

Can I buy the flat frrom the two agreeing sisters and do mutation of the flat with 3part of share and one part for the sister who is not agreeing?

How to settle the dispute?

If it is an HUF property, you can not buy a co-parcner's share, you being an outsider. 

The two other co-sharers can give up their rights in the property by executing a registered release deed relinquishiing their rights in your favor after which you can file a suit for partition and claim 3/4th share in the property with separate possession.

Until a formal partition or a partition by a court judgment you cannot enjoy any  portion of property of your choice because it will remain as unidentified  share only 

Yes, you may buy share of 2 sisters, if they are willing to sell it to you. 

However, you should ensure exact boundaries / demarcation of the shares of all 3 (2+1) sisters leaving behind 1/4th share of unwilling sister.

Has the property been partitioned or, still a jointly held property?

The property do not constitute part of any HUF since all sisters are married and married sisters cannot form part of parental family.