It doesn't matter what the aunt thinks. The cut direct IS rude - that is why it is only reserved for extreme situations.
Responding 'like for like' seldom brings the consequences we want. That brief pleasurable moment from 'putting someone in their place' fades quickly, leaving little of actual value in its place.
If the consequences are "achieving peace by not having a toxic person in one's life," this would appear to be of great value.
The Cut Direct is a necessary part of etiquette. There are (fortunately, rare) people whose behaviour is so horrible that one cannot carry on a civilized relationship
with them. The Cut Direct allows people to simply act as if the egregious person is not there. This avoids the drama of having to interact with them, and makes life quieter and easier for everyone.