Would building a floor for her son and marrying him in it be considered a sin or an injustice to her daughters when dividing the inheritance, especially since the son does not have money to buy a house, the mother does not have money to build for each daughter, and the husband does not care for his children?

It is permissible to enable the son to live with his family on one of the house's floors without owning it, due to his need and financial inability to acquire his own home. To alleviate his sisters' fear of losing their rights, the mother should not transfer ownership of the house to the son. Instead, she should write a rental contract for him, and he may live there for free if he is unable to pay the rent. The son should also write a pledge acknowledging that the floor is not his property and committing to enable the heirs to claim their rights from the house.