From what you've posted, it doesn't sound as if you and your boyfriend spend an obscene amount of time together. However, he doesn't seem to be spending much time with his little brother, either, so I can understand why the little bro might be upset.
J shouldn't take his frustrations out on you, but I honestly don't think that getting a lecture from his big bro about the situation is going to help matters. If anything, it's just going to make J resent you more. From J's perspective, you will be the "bad guy" because you've run to D and complained about J's comments. In the end, J will feel as if D has once again picked you over him and matters could get worse instead of better.
My chapter always had the "sisters before misters" policy, and I know that most fraternities also emphasize brotherhood over girlfriends. It's a safe bet to say that J's feelings are probably being echoed by other members of your boyfriend's chapter.
My suggestion would be to sit down and talk to your boyfriend about his current time commitments. Together, take a look at his schedule and see if maybe he is neglecting his brothers (and particularly J) without realizing it. It could be that he isn't mentioning all the work hours that he's putting in at his dad's garage to the rest of the fraternity. And, if they always see you together on campus, they may have the wrong perception and think that it's you taking up all of his free time instead of a job.
When your boyfriend visits J and/or his other fraternity brothers, is it usually with you in tow or does he go alone? If the answer is that you and he normally visit the guys together, then it's probably a good idea to send your bf on solo missions to the fraternity house for a while. Give him some guy-bonding time, and let J get a chance to hang out with his big brother one-on-one. I think that will help ease the tension between you and J a little.
Good luck and keep us updated!
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