My DH is a middle child and sometimes I tease him about it, but it's true that he's treated very different than his siblings. The oldest is the name sake and the 'smart one' even though he hasn't graduated from college and still lives like he's in college (he's in his 30's) with the drinking and partying. The youngest is catered to by his parents for every whim and every time DH calls his parents the conversation inevitably turns to his younger brother and his accomplishments (real and imagined). DH is quiet and a people pleaser, but I'm not and I won't hesitate to call them out when they're putting DH down. He works hard, supports us while I'm in school, is a great dad and tries to call his parents regularly but he'll never live up to his siblings no matter how hard he tries. It's one of the reasons I'm glad we live far away; it minimizes the drama. Funny thing is, he's the first one they call whenever they need something.
My sister is the quintessential middle child. Nothing is ever enough and everyone else always gets more in her eyes. Funny thing is she's living in a house bought for her by my parents, and they pay most of her bills too. They certainly never helped the rest of us like that, but she still feels that she's the ignored one.
She is loves her big sister more than anything and she tolerates her little brother, but she is the sweetest thing in the world.
I tell you, they are like a knife in the heart from beyond the grave. As a mother, I could not believe another mother wrote that way about such a wonderful child. My husband, a devoted son, was terribly hurt. As you said, nothing he ever did, no matter what he ever accomplished, would ever be as glorious in her eyes as what his older and younger siblings did. So sad. 
