Is what the mother did by selling and buying a portion of her property to her two daughters equally, and documenting it in the real estate registry, considered impermissible (haram) or permissible (halal)? And does that affect the inheritance of her other sisters after her death? And will she be held accountable for it?

What your mother gifted you in terms of money while she was healthy and of sound mind, and you took possession of it before her death, then it belongs to you. There is no blame on her if she dealt justly between her children. However, if you did not take possession of the gift and did not receive it before her death, then it is to be returned and becomes part of the inheritance, based on Abu Bakr's saying to Aisha (may Allah be pleased with them both): "If you had taken possession of it or received it, it would have been yours. But today it is an heir's wealth, so divide it according to the Book of Allah." Merely writing down the gift and registering it in your names does not constitute ownership until you take possession of it and dispose of it as an owner would. There is no sin on the giver unless her intention was to deprive some heirs. And if the purpose of the gift was for it to transfer to you after her death, then it is a bequest to an heir, which is not permissible except with the consent of the heirs, based on the Prophet's (peace be upon him) saying: "Indeed, Allah has given every rightful person his right, so there is no bequest for an heir."