S.H.
I really can feel the NUTSO feelings... because I and all Moms feel that at times.
Its okay. But still it drives us nuts.
I have 2 kids. My oldest is 7. What I do with her is, I always explain to her... about baby/kid development. In ways she can understand. I say, "your brother is just a baby, he cannot talk yet...but he loves you. Look, he smiles at you..." etc. I explain to her... that she was a baby once, and she was that way too. When my youngest is "irritating" to her... I explain, again, about child development to her. She UNDERstands. Then it makes her little brother's actions more palatable. I tell her, "he's at the stage where he does not know how to share.... its not easy for him....", or "He's at the stage/age where he is understanding jokes & doing it... so bear with him..."
All along... I explain to her... about her little brother.
It makes it TONS better for her.
My 2 kids are close and enjoy each others company.
But it takes explaining to them, both... they don't know it, automatically.