I'm the youngest of six and I'm a 4w5. There's a four year span between me and my closest sibling. I think it contributed to my feeling of disconnect. They were this big established group before I came along. I always felt like I showed up late to the party. And then they all left for college so pretty much every year for most of my childhood someone was abandoning me.
I'm an only child, enneatype 1. I'm still not entirely sure of my wing, which annoys me, but my variant is probably sp/sx.
My father was generally supportive, but very often away on business. My mother was a stern disciplinarian and somewhat overbearing; I knew when I'd done something right because instead of criticizing me, she would say nothing. I remember getting a B+ on a science test once, and she asked me what had happened (meaning I had failed). I've really only scratched the surface here, but I don't want to come off as angsty - it just makes sense to me that this contributed to the development of my one-ness. My parents, by the way, are most likey 5w6 (father) and 2w1 (mother).
I'm the youngest child of two. 6w5, phobic sp/sx (not quite sure of instinct stackings but think that's it).
My older brother (four and a half years older) is probably ISTJ, probably a 5, not sure of wings, probably sp-first.
My mother is probably INTJ 5w4 sp/sx.
Dad is IxFJ and hmmm...I think he's either 6w5 like me, or a 1, not sure of wings. Probably sp/so or sp/sx. He's a lot like me. It almost wouldn't surprise me if we were both 6w5 sp/sx and maybe even both INFJs, though I am not sure. I think it's just possible he's an ISTJ with strong F, but I really think he's a Feeler.
FWIW, I think my parents genuinely treated us pretty equally but were a little harder on my brother. My brother clashed more with our dad and I clashed a little more with our mom, probably pretty typical. We were good kids, really. I like to remind them that we gave them a much less hard time than many kids do their parents.
I'm the eldest of 4, 6w5 sp/sx as far as I can tell.
I have no idea on my siblings' enneatypes, though. I know too little of both enneatyping and their personalities.
If I were to hazard a guess,
2nd eldest - 5, maybe 4. possibly even 6.
3rd eldest - really not sure, since she's extremely, extremely introverted and anxious (especially socially). possibly a 9 since she's extremely conflict-avoidant. maybe 6 since she's terrified of everything.
youngest - whatever type fits the ExFP temperament best. 7? 8?
I don't think birth order has too much influence on a person's enneatype, although I do kind of see how being the eldest may cause one to develop 8-ish traits. I'm not an 8 but I am the only one of my siblings with 8 in their tritype as well as being the only one who is sx first. I think being the eldest may have played a hand in being the only one to develop(I believe we develop our personalities and are not just born the way we are)an 8-fix; I had to learn to be the assertive, decisive one, and being the eldest is probably also why I've grown up to be kind of bossy.