Why won't my brother understand...?

Your brother is most likely deep in love with this girl and he isn't gonna listen to reason until he actually sees it himself. Your mom is gonna have to stop with the money giving at least tone it down the with the amount that she gives him. When he gets back from basic he will probably be a new kind of person as well. So maybe the relationship won't last that long when he returns. ( i know from experience). When he comes back from bootcamp he will become a better person and alot more respectful to you and your parents. Best of luck