Solution: According to the fact that my brother's age four years ago is equal to my sister's age six years later, we can know that my brother is 6+4 = 10 years older than my sister.

Judging from the age of my brother four years later and my sister three years ago, the sum of the ages of my brother and sister is 33-4+3 = 32 years old.

So the elder brother's age is (10+32 )÷2=2 1 year.

Sister's age is 21-10 =11year.