Question

Berakhot 19b, at the end of the page:
“Come and hear: ‘And for his sister’—what does this teach? If someone was on his way to slaughter his Passover offering or to circumcise his son, and he heard that one of his relatives had died, one might think he should go back and become impure. Therefore it says: he should not become impure. One might think that just as he does not become impure for them, so too he does not become impure for a met mitzvah. Therefore it says, ‘and for his sister’—for his sister he does not become impure, but he does become impure for a met mitzvah. Why? Let us say: ‘There is no wisdom, no understanding, and no counsel against the Lord.’ There it is different, because it is written, ‘and for his sister.’ And let us derive from it that passive omission is different.”
 
It says that if a person was going to slaughter his Passover offering or to circumcise his son, and then one of his seven close relatives died, then according to the view we mentioned he should not become impure. So just as he does not become impure for one of the seven close relatives, maybe he also should not become impure for a met mitzvah? Therefore Scripture says, “and for his sister” — for his sister he does not become impure.

Now my question is: suppose his sister died and he is the only relative she has—so if not for him, she would be a met mitzvah. It comes out from here that he would not bury her because she is not a met mitzvah. But now because he does not bury her, she supposedly becomes a met mitzvah, or something like that?!