what explanation does third brother give for getting on better with adeline?

I think Third Brother feels a sense of what it is like not to be a favoured child. He is the only person to explain to Adeline why she isn't liked by her other siblings,

"It all stems from our mama dying when you were born. Big Sister and our two older brothers knew her better than I did. I only remember her a little. Things were much nicer when she was alive. You made her go away."