I (20F) am having some trouble deciding what to do about my dad and my brother (16M). For backstory, my brother, whom I'll call John, is in high school, and he currently has a girlfriend, whom I'll call Anna (16F). The issue is that my parents don't think Anna is a good influence on John and think they should break up, and honestly, I don't disagree with them.
Since John started dating Anna, he's spent a lot less time with his friends and has seemed to grow apart from them a lot. His grades have dropped a bit, and he no longer takes any of his extracurriculars seriously (he used to be really driven, but now he does nothing). All he seems to care about is Anna, and I'm starting to get concerned about the amount of time he's spending on her and neglecting his other activities and friendships.
He has spent a LOT of money on her. They're both in high school: he works at a grocery store on the weekends, and she doesn't work. He constantly buys her gifts and goes overboard for special occasions (promposal/Valentine's Day/etc). While I think it's really cute that he wants to do these things for her, it's incredibly poor money management.
The straw that broke the camel's back was prom. They have their prom tonight, and when I called my dad earlier today, he told me about John's plans. I asked John if they were going to meet with their friends for pictures or dinner, and he said two groups had invited them for dinner and pictures, but Anna didn't want to go to either. So instead of going in a group, which is what most people at our school do, he and Anna are going to a (really) fancy restaurant. John also made Anna's bouquet, and he spent over $50 on flowers, and ended up with a lot of extras (you can generally get a good bouquet around here for $20). Anna had been invited to make her bouquet with her friends (which would save money and flowers because they could split bouquets, as most girls tend to do), but she declined so that John would buy and make her one.
From my parents' and my perspective, it seems like John is the only one bringing anything to their relationship. Now, after all of this prom stuff, my dad asked me to talk to my brother about making smarter financial decisions and focusing more on school and his extracurriculars when I get home for the summer. He started off joking that he'd pay me if I got my brother to break up with her, but then he was lowkey serious and said he'd give me $100. I laughed and said that was crazy, and then he kept upping it until it got to $500. I told him I can try, but I don't know if it would work. John is honestly terrible at self-reflection and accountability, and I don’t see him doing anything but getting defensive if I bring up anything.
Anyways, so sorry this is so long, but it's been going on for a long time. My question is, what do I do? I don’t want to just try to make my brother break up with Anna so I can get $500, but at the same time, I think he needs to start making smarter choices. Also, there's more context/other issues (especially with her family), but for anonymity and length, I tried not to include everything.
Edit: just for clarification. I'm not going to sabotage their relationship or do anything sketchy. I just need help deciding whether or not to talk to my brother, and how to approach it. Also, I only get the money if/when they break up, and that's not really my point of concern. My concern is that their relationship seems unhealthy. My dad's offer just got me thinking about it more.