i think you got your answer. Dont convert your sister. Its just not atheist to do so. We atheist only counteract arguments by other religions if they dont make sense. In which, most of the time, never do. Buddhism is one of the religions we may agree with some of the statements, but since atheist can make up their own mind, everyones response is different.
It may counter-act as well, turning her away from atheism if you actually do something to. If shes disinterested, shes got better things to do by the look of things. Just stay the way you are. If you show a good example as an atheist, she may think that it is possible to live a positive life and not follow a religion.
Just leave her alone and live happily that it DOESNT matter whether your sister is of another religion, she is still your sister.