Laura D's article clearly states what being unwanted is. It's when you have to watch your younger and older sibling get what they want. It's when you're left out of the word "family". In fact, the definition of "unwanted" is being the middle child.
As the middle of my family, I constantly have to put up with my parents siding with my little brother. If he takes my belongings without asking, my parents would always say, “You shouldn't have left it there." Is it my problem that he is old enough to reach the top of my dresser? As for my eldest sister, she constantly complains to my parents about how "annoying" I am. Again, they side with her and tell me to stay out of my sister's way. What can I do?
Of course, there are some perks to being the monkey in the middle. For one, you don’t have to be the responsible and mature one, like the eldest sibling. They would always have to set examples for the younger ones. Another benefit is not being the youngest. If you were, parents would never think you’re mature enough to do anything, since you’d be the “baby”. So to all the middle children out there: you’re not entirely invisible.