I want to ask you a question. Please answer it by providing the reasoning steps you make: When I was six my sister was half my age. Now I'm 70, how old is my sister?
When you where six your sister was 3 years old.
Which means that your sister is 3 years younger than you.
Now you are 70, and because your sister is 3 years younger than you your sister is 67.
However it depends when you are born and when your sister is born it might be that your sister is 68 if your sister already had her birthday for the year and you did not. Opposite if your sister did not have her birthday for the year but you had yours she would be still 66.