We have just had our first DC and have a solictor's appointment to write a will later thsi week, so are in pretty much the same situation.
I have chosen not to ask my sister as she lives abroad, and I think it would be too upsetting for DS to loose his parents and then have to go and live in a family where he doesn't speak the language.
All our parents/uncles/aunts are in their 60s so that is unfair on them - imagine looking after a teenager in your 80s!
One of my brothers works offshore, so also out. We have chosen-
DH's brother - DS would be close to grandparents & with close family. DH's bro has a two year old and another due very soon, so are in the same kind of age-range groove. Although DH and his brother are not that close, they share similar views, ideals and interests.
A very good and longstanding friend - Has the same "mind set", philosophy and ideals as me and I trust and love her. She has a 3 year old and is also expecting soon.
My other brother - Not first choice as he is a lot older than me and his children are teenagers now, so not fair to ask him to pick up with a younger child once his are grown, BUT DS would have the most similar lifestyle to what he will have with us - we share interests, are both scientists, shared background and again, the same rellies.
Good luck with your choices. Everyone I asked was very flattered. Although havn't quite got the guts to tell my sister I won't be asking her....