I'm the oldest, my brother and I are both ISTPs (at least I think he is) and my parents are ISTJs (they did the Keirsey sorter in PUM2). Being the oldest and having been raised by ISTJs I seem to have some ISTJ tendencies but I'm mostly ISTP. I prefer to avoid responsibility but if I get tagged with it I wouldn’t feel good about shirking it. My brother, however, just does whatever he wants whenever. Very irresponsible P. He’s also very stubborn. When we were little he liked pushing our parent’s buttons and no matter how much he was punished he would not give in. He was always throwing tantrums. I on the other hand, sometime very early, developed a pathological fear of getting in trouble.
I only have a younger half-brother (although his legs are indeed attached to his torso) on my mom's side, and I'm guessing he's an ISTP, or definitely Ti dominant, because he's analytical, nitpicky, an avid reader and knowledgeable about technical things, but he seems to be a more detail person -- interested in putting things together -- implying Se auxiliary.
Oops! Yeah, I didn't read the directions thoroughly and voted wrong as a result.
“There are two novels that can change a bookish fourteen-year old’s life: The Lord of the Rings and Atlas Shrugged. One is a childish fantasy that often engenders a lifelong obsession with its unbelievable heroes, leading to an emotionally stunted, socially crippled adulthood, unable to deal with the real world. The other, of course, involves orcs.” ~ John Rogers
Umm, in the Birth Order Book they're referred to as only and oldest children. They both had O's. I realized that Only could be replaced with exclusive and the problem was solved. No more effort had to be expended to solve the problem.
I've never put much thought into the eldest-youngest child theories, I guess cause I didn't really see it in real life.... And it always seemed like it was being pushed on me by youngest children- those status-quo-upsetting whippersnappers.