Is it permissible for a mother to take from the salaries of her orphaned children to buy a plot of land for herself and for them, with her brother sharing in the cost of the land?

The questioner is not permitted to dispose of her orphaned children's money unless she has been appointed as a guardian by their father in his will, or by a judge, or by the competent authorities. Her actions must be in the best interest of the children. If there is no will or guardianship, the matter should be referred to the Sharia (definition) court to appoint a guardian to manage these funds.