This is where you need to employ strong interpersonal boundaries with your boyfriend and your brother. Neither will like it -- this I guarantee -- but this is your only option as I see it. And, by boundaries, I mean, interpersonal boundaries. Ex. Your boyfriend is not allowed to vent to you about your brother, and vice versa. That boundary removes you from their conflict with each other.

Why is it your responsibility to be the peacemaker between them? It's not. They are grown men. They can either talk to each other and work out their conflict that way, or be immature and refuse to take the high road.

This will be hard for you to do, if you have any codependency triggers and are a people pleaser. But you have to verbally tell your brother and your boyfriend that you are no longer available to them to vent to about each other. There is literally no other way to remove yourself from the middle, except by literally, removing yourself from the middle.

You can care for them both, so I'm not telling you to stop doing that. But what I'm suggesting is, that you have to set some very clear boundaries here with each of them so that you can enjoy your relationship with your boyfriend and your brother separately.

If your boyfriend refuses to respect your boundary, then you need to question why you'd settle for a romantic partner who gets to dictate when/how you set boundaries with him that are meant to protect your mental health. Just my two cents from experience with this type of situation.