A starting point involves her understanding why this bothers you, or at the very least, caring that it does. Barring either of those, how or why would her behavior ever improve? Sure, she may change on her own, but honestly, conscientiousness tends to be a pretty fixed personality trait (says the woman who still can’t keep track of her phone).
And you’re right that although this may be the only thing you argue about, it’s a large thing indeed. Once you share a life, a home and possibly children, then how mindful and attentive you are to the trappings of that life is a continual and significant thing to disagree about. So, if she doesn’t change, then ask yourself: Could and would you change toward tolerating it?
Hey, bro, about that money …
Q. While helping my parents with financial planning, I learned they’ve been giving my 25-year-old brother money toward his expenses in graduate school. My older brother and I worked hard to pay our own ways through grad school. I expected my younger brother to as well and am disappointed he’s taking my parents’ money, knowing that they should be focusing on their retirement goals. He has also taken loans from my older brother and me, which have gone unpaid. We know he’s used the money to travel and other nonessential expenses. I want to discuss this with my younger brother but don’t feel it’s my place to judge, since my parents paid for the majority of my more expensive undergraduate tuition. —Brotherly Frustration
So, discuss without judging (or as we shrinks would say, “explore”). Start with his overall plans; if he’s got unpaid loans from you, it’s your right to know. Say that since you’re helping your parents with financial planning, you need to know where things stand in the long and short terms.
Talk to your parents privately about their financial goals, and how their “expenditures” (ahem!) may get in the way. But you must decide, truly, if this is really about your parents’ financial future, or if it’s actually just resentment about his not paying for grad school like you did. The former is a reasonable and viable conversation; the latter, though understandable, can veer quickly into “Johnny got a bigger slice of cake!” territory.