You're now 6 and your sister is 3.

When you'll be 63, 57 years will be gone already.

So your sister will be 3+57=60 years old

Sister is 3 years younger than you.

When you are 63 she will be 60.

You are a very precocious 6 year old to want to know the answer to a question like this.

I'm 6

My sister is half my age

My sister is 3.

There is a difference of 3 years

When I'm 63, my sister will be 60.

60

If you're 6 and your sister is half your age that means she's 3. Right? That means she's 3 years younger than you, 63 - 3 is 60.

sixty