You can make use of equations.

For instance, let the number of years you need to grow to become 100 years old be x. Then

4 + x = 100

x = 100 - 4 = 96

When you are 4 years old, your sister is half your age, that is, 0.5 x 4 = 2 years old.

Your sister grows for 96 years to become 2 + 96 = 98 years, when you are 100 years old.

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.**