Basic Trope: The middle child is favoured the least.
Straight: Alice and Bob have three children from the oldest: Charlie, Daniel and Ethan. Daniel is neglected the most.
Alice and Bob have seven children from the oldest: Charlie, Daniel, Ethan, Flower, Gerald, Hillary and Igor. Flower is neglected the most.
Same as above, except only Charlie and Igor get attention. (Igor could be a Magical Seventh Son.)
Alice and Bob have lots of children. The one right in the middle of the birth order is neglected.
Downplayed: Alice and Bob have three children from the oldest: Charlie, Daniel and Ethan. They treat them with love and care; just that Charlie and Ethan gets more attention than Daniel.
As the oldest, Charlie will be taking on Bob's legacy. As the youngest, Ethan gets to be pampered and babied. Daniel is neither, so naturally, he is neglected the most.
Daniel is average but Charlie and Ethan have some combination of:
A) Charlie and Ethan have learning difficulties. B) Charlie and Ethan are physically disabled. C) One of each. D) Some other combination that logically results in a necessity of parental attention skewed away from Daniel.
It's not unfairness - just a fact of life.
Inverted: Daniel is pampered and supported the most, while Charlie and Ethan are neglected.
Subverted: Daniel complains about this and the counselor confronts his parents about it. Alice and Bob claim otherwise.
Double Subverted: Because as far as they're concerned, they only have two children.
Zig Zagged: Alice and Bob have seven children from the oldest: Charlie, Daniel, Ethan, Flower, Gerald, Hillary and Igor. Charlie, Flower, and Igor get attention, while the other four are neglected.
All three children are treated equally; that is, they are all either supported or neglected.
None of the characters have more than one sibling.