A lot of issues I had as a kid, I used to chalk up to being adopted. But my self-dx as Aspie really offers an alternate explanation. Being adopted prolly just exacerbated things.
I am the oldest, and have a younger brother (also adopted). Our parents definitely favored him, partly because he was younger (parents often are easier on the second child than the first) and he was a boy (I'm female). But also because he had a mild case of Tourettes, and although he didn't have grand mal seizures or anything and could play with other kids, make friends, and was a jock in jr and sr high, my mother doted on him and spoiled him rotten because "he's a sick little boy." I would even get punished sometimes for things HE did.
Meanwhile, they had high expectations for me but were never satisfied - I was definitely NOT the precious little princess my mother hoped she was getting from the agency. I was a tomboy and an early reader and very precocious, asked too many questions, exhausted her with my chattiness on certain subjects, my love of nature and bugs and other critters, and my emotional "neediness" (I sensed her recoiling from me and was hurt by it), said I thought rules were "for other people," etc.
In college, my dad would mail me newspaper clippings of my brother throwing a winning pitch at a ballgame, but he (dad) never found the time to visit me or call, or find anything praiseworthy to say about me (that made it back to my ears, anyway). I'm in my late 30s now and have not heard from my "dad" in 3-4 years now. I guess I'm too much trouble, and he found replacement step-daughters.
So, yes, I was extremely jealous of my mostly-NT, average-intelligence brother. I also tormented him when we were very young - for which I have deeply apologized to him for, repeatedly. Of course he claims he doesn't remember. But I do.