<s>[INST] When I was four, my sister was half my age. Now I am 28, how old is my sister? [/INST] When you were 4 your sister was 2 years old, since she was half your age. That means she is 2 years younger. Age difference always stays the same. So, to find your sister's age we need to subtract 2 from your current age(28).
28-2=26
Your sister is 26 years old. </s>