Not sure whether you are asking how long to boil his bottles, while sterlizing, or how long (in terms of age) you should be boiling them for.
I will try to answer both questions.
My dh did some research and found out that you should boil the bottles for 15 minutes for effectively killing the germs, while sterlizing.
In terms of age, your sister is right; You can stop once they start putting things in their mouth. Some people even pop the bottles in the dishwasher. In our case, we just decided to sterlize for the first year of our dd's life, because she is more prone to infections.
HTH.
Sumita