my hubby has a sign in his office:
"ignorance can be cured, but stupid goes clear to the bone."
it's too bad that some folks never learn to just admire, and not to criticize. it might have been different if the "cousin" was an established fabric artist in her own right--but i suspect that if a fabric expert did approach you, they would do so with finesse and admiration of your skills, not as the "quilt police".
sounds like you should get a medal for good behavior under undue pressure!