One idea would be to just let this go and focus on yourself. And on yourself and your boyfriend. Sounds like the brother is into controlling, maybe he is the alpha in the family, and he is keeping you and your boyfriend in "your place". Which is ridiculous, but you can take the high road, and not let his games get to you.

What does your boyfriend say about all this?

You've asked to be included, the brother won't include you, which is unnecessarily mean on his part. Maybe he is working to stay in charge and in control. So you can step outside the game he is playing. You can instead stay busy with other things and with people who reasonable and kind.

He may want to goad you into being upset so he can then claim that you are the "crazy one". Don't let him do that.

Take your own power back. Remember that if the brother purposefully excludes you - it is his loss. He is missing out on an opportunity to know a good person who loves his brother (your boyfriend) and who could make a positive contribution to his family.

Maybe the brother feels threatened and he is protecting his turf. Or maybe he is just a jerk. No matter what, don't let him goad you into being upset.