Do extra chores, such as cleaning up. loading the dishwasher or just helping out around the house... of course it depends how generous your parents are.
use your money properly
Yes.
By mowing a yard or hustling your parents. (that's What i did at that age)
if he mows lawns or shovels snow.
Make a lemonade stand.
The boy is 9 years old and his sis is 18 years old
If the boy is four years old and his sister is three times as old, then she is 12 years old. The age difference between them is 8 years (12 - 4). When the boy is 12 years old, his sister will be 12 + 8, which makes her 20 years old.
Lemonade stand.
Sell lemonade on a stall.
It is certainly possible for it to happen. As long as the boy has reached puberty, it is possible.
13
Babysit depending on how old they are (12-13 years is about old enough)