I’m very sorry to hear this.
It’s quite possible your parents are trying to “overcompensate” because they perceive your brother as the weaker one who needs encouragement, and yourself as the go-getter who’ll be fine no matter what. In short, they have high expectations of you because they think highly of you, whereas your brother, they think, “well, he’s, well, slow” and immediately feel they need to add something positive like “but he’s sweet” or whatever, because as a parent you try to go easy on the weaker kid.
Still, they probably don’t realize that you, too, need positive reinforcement and affirmation on their part.
I’m not quite sure how to broach this but I hope other parents will.