This is from a TV show.
Emma and P.J. meets at the park. They each take their baby siblings out in a stroller.
Emma said to P.J.,
I'm really impressed that you take your sister to the park. My brother would never do anything like that.
P.J. replied,
Hope not. She is my sister.
What does "hope not" mean?
PJ is misunderstanding the ambiguity of "like that", probably on purpose, for comic effect.
"Like that" refers to
Like PJ taking PJ's sister to the park
So "Like that" means
My brother taking his sister to the park.
But PJ (pretends) to misunderstand it to mean
My brother taking PJ's sister to the park.
So she says "I hope not". She means "I hope your brother isn't taking my sister to the park" because Emma's brother is a stranger to PJ.
