Answer:
Number of brothers is a quantitative variable, because all the possible values that this variable can take are only numbers, it's a parameter that is either counted or measured. For example, the number of brothers can be 10, 2, 5, 7, etc.
Qualitative variables are those that are represented by atributes or qualities, the ones that can't be measured or counted. These variables are commonly categorized. For example, a qualitative variable would be the hair color of each brother or whether a product is defective or not. In a defective product, you can't measure the defect in the product, and the variable can't take numeric values.