Let A be your age, B be the age of your brother and S be the age of your sister.
Since your brother is 1/4 your age, then:

Since your sister is 8 years older than your brother, then:

Since your sister is 12 years old, then:

Use the second equation to find your brother's age by plugging in S=12:

Plug in B=4 in the first equation to find your age:

Therefore, you are 16 years old.