I think you are right she would probably go buy something of equal value and then you two would all be "square" or "ok"...

My ex had this happen with her 3 siblings...meaning the equal thing - always justified the amounts or values of items...had extreme favoritism...my ex wrote a letter...it did little good...she didn't have a clue what my ex was trying to get at...so we both adopted the equal, fair, just right thinking and not compare monetary values between the kids...

Sorry I don't have any other advise other than spread the word...if you agree in this type of thinking....