Something has been stirred up with your boyfriend’s brother. He must have a reason for his initial response to the two of you dating. I’m not saying that his response is accurate, or valid, but we can’t dismiss him as crazy either. If you know why he feels that way toward you, then it needs to be discussed openly with him. If you truly do not know the reason, then this is when you are going to see what your relationship is made of.
So, what do you do? The first step is for you and your boyfriend to decide how you are going to handle this as a couple. Do you want your boyfriend to approach his brother alone first? Is it possible to have an open discussion with the three of you together? This is an opportunity for you to problem-solve with your boyfriend. It’s his role to interface with his brother and be your advocate, while at the same time realizing that his relationship with his brother is also very important.
It is your boyfriend’s responsibility to understand and mediate what is happening with his family. He can’t control his brother’s behavior, but he should be able to talk to him about it and communicate how his behavior is hurtful and inappropriate. If you think his brother is going to tell your boyfriend something about you that you have not disclosed yet, now is the time to be open and honest.
Secondly, recognize the possibility that your boyfriend’s brother is jealous of the time he spends with you, or is afraid that somehow he is going to lose his relationship with his brother. He may not like the influence he sees you have over his brother. Again, it may not be logical, but the best place to start is to validate how your boyfriend’s brother is feeling. Clearly, there is anger and resentment building. The best way to prevent him from being defensive is to hear him out, even if it’s painful for you. Listening first will make it a safe environment for the other person to be real.
Which means: you have to start by being real. I would imagine in addition to being angry, you are also very hurt by his behavior. Maybe embarrassed? Afraid about how it might affect your relationship? How does your boyfriend feel? Has he been able to talk to you about it? I am hearing that you would like for the relationship between his brother and you to be different. Start from that perspective. You are trying to find out why he is upset; address the issue, and talk about how to get along better in the future.
You mentioned that your boyfriend has spoken to his parents about it. I’m assuming you are both young adults, so it’s really not his parents’ job to make sure siblings get along. It sounds like his parents are attempting to avoid a very obvious conflict. Or, it may just be the way your boyfriend’s family handles conflict: denial in hopes it will disappear.
But in the end what is most important is how the two of you as a couple react to this type of stress. Is your boyfriend able to hear your concerns? Can you have empathy for the position he may be in, with his own brother and his girlfriend not getting along? Can the two of you talk about it without an argument? Can you solve this as a couple? If these discussions don’t go perfectly, it doesn’t mean your relationship is doomed. It just may mean that it needs some time to grow and mature.