I'm adopted and when I decided to go looking (about 7-8 years ago), I found a grave, my birthmother had died at a fairly young age (under 40). I had a feeling that due to her early death, it was unlikely that her extended family, eg uncles/cousins, would know anything about my existence because I was born overseas while she was on a working holiday (she had no other living children (she had stillborn twins)) so wasn't sure how to make the approach. In the end, I did find the right approach (which was sort of unique to my situation so won't go into here).
When I got my OBC from NZ, I got a little booklet called "Approaching your birth parents" which you might find of interest because in the last portion of the booklet, it talks about the ins and outs of each approach:
http://www.cyf.govt.nz/documents/abo...ts-04-2005.pdf
Even though it is about contacting birthparents, it would still be of help regarding contacting any birth relative.
(NB: When you bring up the PDF, you will find a blank green page so you will need to scroll down a little to find the actual booklet).