How should a house be divided between a sister's children (a boy and a girl) according to Islamic law, taking into consideration that the first and third floors were built at the sister's expense, and the second floor was built at the expense of her daughter's husband?

If a woman gifts the air space of a house to her daughter's husband for him to build a house there, then the air space and what is built upon it become his property. This is permissible according to the Maliki and Hanbali schools of thought. As for the third floor that the woman built for her son, it remains her property. If she gifts it to him during her lifetime and he takes possession of it, then it becomes his property. However, if she does not gift it to him, but merely permits him to reside in it, then it remains her property and will be considered part of the inheritance after her death.