How does "The Scarlet Ibis" show that the narrator had mixed emotions about Doodle since the boy's birth?

I suggest looking at paragraphs three, four, and five for information about how Brother feels about Doodle's birth and having a brother.  
Paragraph four describes how Brother is very excited about the possibility of having a brother.  A brother would give him a buddy to play with.  Brother tells readers that he desperately wants somebody to run, jump, and climb stuff with.  That makes perfect sense.  Brother is six years old, and he's really active.  When Doodle is born, Brother can't help but be excited about finally having a playmate.  
That excitement is mixed with disappointment though.  

He was born when I was six and was, from the outset, a disappointment.

Doodle is physically disabled, and the family suspects that he is also mentally disabled.  Brother has built up in his head an ideal of what having a brother will be like, and with Doodle's limitations, Brother realizes that his hopes have been dashed.  Brother even admits that he is willing to accept Doodle's physical limitations, but he is not willing to accept the mental limitations.  

It was bad enough having an invalid brother, but having one who possibly was not all there was unbearable . . .

That's why Brother plans to kill Doodle.  That's dark; however, those feelings of doom and gloom are immediately replaced with joy and hope once Brother realizes that his parents are wrong.  Doodle is mentally just fine.  

However, one afternoon as I watched him, my head poked between the iron posts of the foot of the bed, he looked straight at me and grinned. I skipped through the rooms, down the echoing halls, shouting, "Mama, he smiled. He's all there! He's all there!" and he was.