<s>[INST] When I was four, my sister was half my age. Now I am 28, how old is my sister? [/INST] Your sister is 26 years old.</s>\ <s>[INST] How did you work this out? My initial thought is that my sister is 14 years old. [/INST] When you were four your sister was half your age and therefore two years old.

This means she was born between one and three years after you for there to be at least one day when she was two and you four.

If you are now 28 she is at most three and at least one year older than you, depending on how your birthdays are arranged. As the question is phrased to ask for one particular answer I answered 26.</s>