Upside to my sibling situation: I got one of each, one brother and one sister. Downside to my sibling situation: I am the middle child, and quite the textbook definition of it I might add. My parents had their own interesting sibling situations. My father grew up as the only boy (mostly), having only sisters and was the eldest. My mother grew up as the only girl, she only had brothers and was the youngest.
My belief is that who we are is made up of many factors that work together as a formula. Where you lie in the sibling spectrum (middle, eldest, youngest, only, twin), your gender in the spectrum, the time of year you were born (I'll come back to this one in a bit), and the distance in age amongst one's siblings all shape and work their magic into this formula.
I am a middle child where I am far apart from my siblings in age. I am closest to my sister because we are closer in age, and the majority of time, it was just the two of us. By the time by little brother was born, my sister was already a teenager. So even though he is our brother, it comes natural for my sister and me to be parental towards him. To give you some more perspective about our age differences, the time I graduated with my master's degree was the same time when my brother graduated from high school. My siblings also have textbook components to them. My sister, being the only girl, and who was the only girl for some time, was treated as such. Hence her strong personality and easiness to clash with like minded people, there's only room for one alpha dog. My brother, being the youngest by large gap and needing more assistance at early elementary age, was treated completely differently from my parents' prior children. The reason my sister and I feel parental towards his is because we had to take on parental roles. While my parents worked, my sister played a big part in taking care of my brother as an infant, while I worked on academics with him when he got to be of grade school age. My brother, as such, has developed a personality which fits that, one where he feels relaxed because things will work out as there are people looking out for him. I grew up where I was the youngest for eight years, so I did have my moments of being treated as such. But the game changed, and even more drastically for me, when a younger sibling arrived, we had moved residence, and started living in a new neighborhood all in the same year. It was only shortly after that I knew my responsibility for someone else was going to skyrocket. So I feel the early years shaped my extroverted nature like my sister's while the twist in my childhood contributed to my constant need to feel that if I'm not responsible or organized, that the world will implode. Because of our early on experience, my sister and I can act the same and have the same tastes in humor, like with our inside jokes, while my brother has more of an introverted innocence to him.
When it comes to my choice in friends, I tend to gravitate towards people who are the eldest mostly because they have that understanding of responsibility, all the while sharing some of the same extroverted traits. Not to say some younger and only children haven't snuck their way into my life, as there are always exceptions.
I also feel a pull towards people born around the same time of the year, even of the same astrological sign (which can be hokum, but I'm not going to be discussing that). And by coincidence, I've also met a lot (and I mean A LOT) of people with the same birthday as mine, which has it's own air of weight as it is, 9/11 if you must know. There's something about relating to people who more than likely started school as four year olds, were one of the youngest in their class in every grade after that, and went to college pretty young that makes one have to grow up faster than their older counterparts (Who goes to college at 17? Oh wait, I did). As a result, that sense of responsibility is raised, which only adds to he level of responsibility I already gravitate towards. I know this part strays away from the sibling concept I'm exploring, but it's important in formulating who I have become.
My siblings and I get along great. I think the age gaps helped in that there was never a time we genuinely felt "sick and tired" of one another as we were always in different phases of our lives. When one was a teenager, another was playing with Ninja Turtles, and the other was literally a fetus. This marked our relationship in terms of friendliness with another in that we have been apart enough to appreciate our time together.
On that note, I will advertise that we are participating in the Walk Now for Autism Speaks event at the Rose Bowl this April. More info here: http://www.walknowforautismspeaks.org/faf/search/searchTeamPart.asp?ievent=1124950&team=6246899
Before I go, shout out to my sister will soon be celebrating a birthday. Here's to another one! And to my niece, best of luck with your big sister-ness of the twins.
Afterthought: I was thoroughly amused by feedback my previous post generated.