Is zakat obligatory on a sum of 15 pounds weekly from my mother?

Zakat is not obligatory on the mentioned amount unless a full year has passed over it and it reaches the nisab, by combining it with any money or trade goods the questioner possesses. The nisab is equivalent to 85 grams of gold or 595 grams of silver. The amount due is 2.5%, and the year begins from the day the wealth reaches the nisab.