Is the sale made by the cousin of the aunts' shares in the joint house, without the knowledge of the other heirs and without giving them the opportunity to buy, religiously valid? And are the uncle and the cousin sinful for not informing the heirs before concluding this contract?

If a group of people share ownership of land or a house, it is not permissible for any of them to sell their share before informing their partners. This is based on the hadith narrated by Jabir: "The Messenger of Allah (peace be upon him) ruled with pre-emption (shufa'a) in every partnership that has not been divided, whether it is a quarter of a house or a garden. It is not permissible for him to sell until he informs his partner. If he wishes, he may take it, and if he wishes, he may leave it. But if he sells without informing him, then he (the partner) has a greater right to it."

Therefore, if the paternal aunts sold their shares to their nephew without informing their female partners, then the female partners have the right of pre-emption (shufa'a).

If more than two partners share ownership of a property, and one of them sells his share to another partner, the remaining partners have the right of pre-emption according to the proportion of their ownership, as mentioned by Ibn Qudamah in "Al-Mughni."

Accordingly, the share of the paternal aunts that the nephew bought should be divided between the female partners and him, according to each of their ownership proportions in the shared house.

As for the remaining paternal uncle, he has no right of pre-emption because his agreement to the sale and his failure to claim pre-emption is considered a waiver of his right. The right of pre-emption is established immediately; if you delay in claiming it, your right is forfeited.