Is the sisters' lack of objection to their brother and his children benefiting from the land considered an affirmation of the unwritten Mugharasah contract, and how can the land be divided in this case?

A mere permission to utilize the land without charge is not considered a Musaqa (cultivation) contract; rather, it is a pure act of generosity. A Musaqa contract requires offer and acceptance.

There are two forms of Musaqa: division of land and trees, or division of trees only.

Musaqa based on partnership solely in the trees is permissible according to the Hanafis and Hanbalis, but prohibited by the Malikis and Shafi'is.

Musaqa based on partnership in both the land and trees together is permissible only according to the Malikis.

If the maternal uncle's sons provide evidence of a valid Musaqa contract for a known percentage of the land and trees, then, according to the Maliki school, this percentage transfers to the maternal uncle's heirs.

The best way to resolve such disputes is to resort to the Sharia court or to consult with scholars in the locality.