( Source : maharera.mahaonline.gov.in) Yes, the layout of the real estate project has to be approved. However, the promoter may include some buildings in his application of registration where apartments are proposed and the Building approvals are pending. Building Approval for the apartment...
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Once the Gift Deed is registered and if the flat in question was acquired by your father from his self earned, income, then your sibblings would not be entitled to claim any share in the property.
You can file a suit on the basis of adverse possession before a court of competent civil jurisdiction and seek ownership of the property on the basis of you being in exclusive possesion of the property for over 12 years.