I understand where you're coming from. I had my siblings ruin my property in the past and my parents didn't give a shit, and when I told them they would have to replace it since they ruined it, my parents were like "they don't owe you anything. They didn't mean to break it" -_- and just stupid excuses like that because they're favored over me. Anyway, your brother should replace your headphones, accident or not, he didn't ask to borrow them so he is responsible for the damages. Whether it sounds "childish," I would just say take his money and buy yourself new earphones. If he's not gonna get them for you, get them yourself...with his money.
Come on, if you're old enough to recognize something as "childish" but you still advise doing it, then you're doing adulthood wrong. Both of you girls need to grow up.
Look at this way: if you just take your brother's money, you're teaching him that revenge is the only way to solve someone taking something of yours, and pretty much asking for this to escalate and you can kiss something else of yours goodbye.