I have two older half-brothers (14 & 18 years older than me) and I couldn't have asked for better people to have in my life - words can't describe how thankful I am to always have these guys looking out for me and supporting me. but i will try, as evidenced by this very long post ahead, sorry
Because our dad is so busy and the oldest of my two brothers was already an adult by the time I was born, he often took on the role of my 'dad' in driving me places when I was a kid a lot, and even before that I know he was the one who took my mom to get her ultrasounds and checkups and all that. And the younger of my brothers always took me fun places when I was little; he was also the one I turned to when I found myself in trouble that my parents couldn't find out about since he was old enough to give advice and help without scolding me or anything like that, haha.
Even now they still take time out of their busy lives to hang out with me and take me to cool places whenever they can. Our relationships have changed a bit from guardian/kid to friends now that we're technically all "adults", but they're still always looking out for me. And I know I'll always be the "baby sister" no matter what - I think it was only like two weeks ago, when I was sitting on the couch and watching TV with one of my brothers, and he suddenly hugged me and said "WHEN DID YOU GET SO BIG, BABY SISTER. REMEMBER WHEN YOU WERE THIS LITTLE?" gestures the size of a cushion. :P
I am my mom's only child though, and I never lived in the same house as my brothers so most of the time it's like I'm an only child. I remember when I was younger I wanted a little sister, but mostly only because I wished my parents would not be so protective of me and let me go out more; I thought they would loosen their grip on me if I had a younger sibling for them to care for, haha.
But I still do wish I had a sibling that was closer to my age - my mom's best friend is her sister, who is only four years younger than her and my brothers are also four years apart and really close as well, so I'm a little jealous of that kind of relationship. :)