What about the title documents?, on whose name is the title vested? If your father had purchased it, did he have a sale deed registered on his name?, how did your mother acquire the property on her name after your father's demise?, If she acquired on her own without considering the share of other legal heirs or getting a consent or registered release deed in her favor, such transfer can be questioned and her title itself will be under scanner, therefore if she is not having proper title to the property the gift deed executed by her in your favor will also be considered as invalid. However, as a precaution you may get a release deed from your sister in your favor and get all the records transferred on your name on a mutual understanding with her. If she has not questioned about this transfer so far, you may leave it as it instead of reopening the dead issue.