Question in the G.M. about blessings (mat mitzvah, postponing a mitzvah before a mitzvah)

Blessings 19, page 2, at the end of the page:
Ts and his sister, what is the Talmud to say? He was going to slaughter the Passover lamb and circumcise his son and heard that he had died. He could return and become impure. You said, “He cannot become impure.” Just as he does not defile them, so he does not defile the dead. Ts and his sister. He is not impure, but he defiles the dead. A mother has no wisdom, no understanding, no advice against the Lord. I am innocent, as it is written. And his sister. And to finish her, return and do not do what I am.”
 
It is written that if a man went to slaughter his Passover lamb or circumcise his son and one of his seven relatives dies, then according to the method we mentioned, he will not become impure. Just as he would not become impure for one of his seven relatives, he would not become impure for the deceased. Is it a mitzvah? You should learn to tell your sister that he does not impure his sister.

Now my question is, if, say, his sister dies and he is the only relative she has (if he were his, she would have a mat mitzvah), then it follows that he will not bury her because she is not a mat mitzvah and now because he will not bury her, she is supposedly now a mat mitzvah or something like that?!