I agree with Bill3 100%.

Your boyfriend is not in a relationship with your brother. He's in a relationship with you. How he feels about your brother is moot. If his hidden resentment surfaces as passive-aggressive statements to you, the silent treatment, or "bad moods" then THAT is emotional manipulation on his part.

He needs to get over it and resolve it directly with your brother. If he won't, then you need to decide -- do you want to commit yourself to a guy whose ego is so fragile, he's willing to take out his immature behavior on you (his closest target), which is settling.

Or, can you discern better in the future, and choose a more emotionally available man who may not like your brother, but voices it as concern and then drops the issue, and focuses on nurturing his relationship with you, and nurturing and emotionally supporting you.

Something to think about.