What is the ruling on a simulated contract for the sale of a house belonging to the aunt’s husband, and are the buyers entitled to dispose of it, and what is the ruling on the ownership of the house in this case?

Selling a house in exchange for lifelong support is impermissible due to the presence of gharar (excessive uncertainty) and jahalah (ignorance regarding the exact terms). However, if such a transaction occurred and has passed, it is to be settled by its value. Therefore, if your aunt's daughter built upon the house until it became three stories, she has acquired ownership of it, and its value is binding upon her to her stepfather. She can, however, offset this value with what she spent on it. As for the fictitious sale of the dwelling to you and your brother, it appears to be a gift (hibah) which is not completed except by taking possession (hiyazah). Thus, if she vacated the house and enabled you both to take possession of it, you would own it. But if she remained living in it, the gift is not complete.