I remember reading somewhere that you have the longest relationship with your siblings. Pretty obvious really as you tend to be born around the same time, hang out in a shared building during childhood and hopefully die around the same time. But it's not just about time is it? Siblings share a dollop of genetic material and a very similar upbringing. No one else in my life has that.
If I had been brought up by my parents without my older sister and younger brother present for dilution of parenting and moral support, I am not sure how I would have managed! The adult to child ratio was definitely in our favour. We had a lot of mischief to get on with so decoys were essential. My sister's teenage rebellion provided much needed cover for my brother's and my wrongdoings for years but mostly I think we worked on a rota system.
Of course we had some sibling rivalry rattling about, enhanced by our parents need to continually - and sometimes brutally - compare us and in a family where 'healthy relationships' were not readily modelled for us, I am sure we were pretty mean to each other at times. I do remember hitting my brother over the head with a glass bottle, for example. And I do also remember fighting with him outside school once and using the excuse, 'but he's my brother' as absolute unchallengeable justification and amazed that teachers thought they had any right to intervene with a sibling fracas! I wonder if sibling rivalry can ever be 'healthy' because I'd say us three still got unhelpful twinges up until quite recently.
And then there's the order of birth stuff. I have read the eldest is meant to conform bigtime(jobs like police, pilot), the middle is meant to be the communicator (become a teacher, counsellor) and the youngest is meant to rebel and be most likely to opt out of mainstream. I have also read something that states this generalisation was a nonsense because it was based entirely on sibling perception.i.e. the eldest seemed more conforming to the youngest (their greater age alone might make them seem thus)...not an impartial outsider's view. However, with the exception of my big sister who I have never seen in a uniform (except Brownies - which she eventually got kicked out of) it sort of works a bit for us. My brother is completely 'off the main road' (visiting Venus perhaps). There is a family joke about him driving up to my mum's house in his Mercedes and saying 'hop in mum, I've come to take you out to lunch'. I'm not sure that would be funny to anyone outside our family but it gets me every time! He's a musician still living a very 'free' life in Brighton.
My sister and I joke about a Venn diagram, where we overlap and where we don't and there are undoubtedly areas labelled 'just Molly', 'just Mike' and 'just Claire'. But that three-way overlap in the middle is apparent when we all get together - the overlap that means we have free-flowing fun and a considerable amount of laughter! I cherish that!