Just some ideas - did your brother tease you for being a "baby"? Was he jealous of his little sister taking his mom's attention? I'm not aware of this in my past - maybe the parents made my brother feel ashamed for being jealous, but i would have been aware of that and felt his shame anyway.

I remember one incident when i was surprised that my brother didnt try to take the credit for something i did - i dont remember any time he DID try to take credit, but then why would i so clearly remember being surprised that he did not?

Anyway, i think it might be some transferential shame. Feeling you're not allowed to have it. Who are you being disloyal to if you accept it. Ultimately the answer is - yourself. You have to choose yourself. Love your neighbor AS YOU LOVE YOURSELF.