Is Zakat obligatory for my mother, and what is its amount, if she sells land she inherited from her father to her brother?

There is no zakat on inherited land before its sale. However, if it is sold and a hawl (lunar year) passes on its price, or on what remains of it, and it reaches the nisab (minimum threshold) by itself, or when combined with cash or trade goods belonging to your mother, then zakat becomes obligatory on what a hawl has passed on. The nisab for existing currencies is the equivalent of 85 grams of gold, or the value of 595 grams of silver. The amount that must be paid is two and a half percent (a quarter of a tenth).