My father's been married several times. In his first marriage, my oldest sister (two months shy of 48) and only brother (46) were born. During a fling, my other sister (40) was born -- who happens to be cousin to a good friend of mine since we were four years old (now 39).
Stupidly enough my parents had the great idea to keep away from my half-brother and my two half-sisters for about eight years. I practically grew up alone while seeing my friends with brothers and/or sisters.
At the age of 15, I finally met my oldest sister and only brother. A year or so later, my other sister introduced herself as such whom I had known only as my friend's cousin.
My father was willing to introduce me to my oldest sister (maybe the last good thing that my father ever did for my oldest sister and me; http://project05952381.blogspot.com/2010/10/my-fathers-last-betrayal.html) who pushed me to meet by brother. Then I introduced my younger sister to my brother and older sister. This is no way to meet one's family, but this is the way we finally met and got together.
