I feel like an only child....and technically I am the only child between my biological parents.
However...
1 half brother on my Mom's side that is 5 years older. I grew up with him.
At least 5-6 siblings on Bio-Dad's side. I don't know any of them.
DH has 1 sister (12 years older) and 1 brother (11 years older).
I have an older stepsister, an older stepbrother, a younger sister, a younger brother. I know the younger set far better than the older one, but all four are awesome.
I have one brother but I haven't talked to him in over 4 years and don't plan to anytime in the near future. I'm close to my ex-sister-in-law, his ex-wife though.
I am an only child and I have no cousins either (my dad was an only child and my mum has a brother and sister who have no children.) I have one SIL though now that I am married and will have a BIL soon as my SIL is getting married.