Is it permissible for a mother to sell a plot of land and allocate a portion of its price to marry off two of her children, excluding the others, while keeping the remainder for herself?

The fundamental principle is the obligation to be just among children in giving, and this is the view of the Hanbalis. However, it is permissible to favor some children due to their need, such as for marriage or the like, or because of their virtues, or to withhold a gift from some of them due to their transgression or innovation, as reported from Ahmad and Ibn Qudamah. If some of your mother-in-law's children are in need of marriage, then she may favor them with gifts over her other children.