Hi there,
If you are 6 and your sister is half your age, that means she would be 3.
So, when you are 70, your sister is still 3 years younger than you, which means she would be 67
Step 1: 6 divided by 2 is 3
Step 2: 70 - 3 = 67
Hence, the answer to your question is 67
Hope this helps :)