Your sister is 2 years younger than you as when you are 4 she is 2(half your age); therefore when you are 100 your sister is 98 years old.
When you were 4 years old, your sister was half your age. Half of 4 is 2, your sister was 2 years old when you were 4 years old or she is 2 years younger to you.
Your present age is 100, as your sister is 2 years younger to you, she is 98 years old.
` 4-:2 = 2` so you sister is 2 years younger than you
100-2=98 she will be 98