If Aunt Margaret tells you someone is her "cousin," what does she really mean? First cousin, second cousin, etc.? To the genealogist it makes a difference because first cousins (who share grandparents) are different from second cousins (who share great-grandparents). First cousins once removed are of different generations from the common ancestor--the grandparent of one was the great-grandparent of another.
It's not necessary to confuse Aunt Margaret. Instead of getting her to tell you the precise word, have her explain the way they were related, generation by generation, or ask her about how they were related. You can figure out the precise word later. And asking those questions may elicit more information anyway!