Sibling … Rivalry?

I am one of many.  There’s an Elder, an Heir, an Afterthought, and a Partial.  Elder, of course, is awesome, as an eldest should be.  Mini-adult, little Grown, from early on.  Heir is … well, Heir.  Entitled, selfish, as expected.  Afterthought should not exist, as Partial, but both do, for whatever reason.  Afterthought is one …