When I was 6 my sister was half my age Now I'm 70 how old is my sister?

"When I was 6 [...] Now I'm 70." : The difference is 70 - 6 = 64 years.
"my sister was half my age" : 6 / 2 = 3.
The sister has also aged 67 years. So, she will be now 3 + 64 = 67 years old.

... unless she was travelling for some months at near the speed of light; then she could be significantly younger ;)

67, right?

since when she was 6 and her sister is half her age, half of 6 is 3. when she is 70 her sister would still be 3 years younger than her so her sister is 67. (Been fooled with this so many times)