Oh Chaco, I know EXACTLY how you feel- my first post on this forum was due to the very same thing. DH's Brother and his wife had had their first child (first grandchild for the in-laws) and I got frustrated that I couldn't want a baby like everyone else and I just couldn't go all 'goo-goo' over the new arrival. It doesn't make much sense when you talk about it to other people but on here nearly all of us know how you feel.
We went to visit the baby last weekend and I found it really hard to fake the "awwww"s but I knew I HAD to do it for the sake of the rest of the family. I saw the smiling happy photos of everyone holidng the baby too and felt this pang of jealousy- but to be brutually honest when I went to visit I saw that it was meerly a snapshot- the 'kodak moment'. The reality was that Mom looked stressed, tired and bored and she admitted to me that she has been getting teary most days (sounds like PND to me). All is not always as it seems in the photos.
As for babies bringing families together, they can also cause rifts that weren't there before. My sister-in-law has pulled out of being a bridesmaid for my other sister-in-law since the birth, which has obviously caused a bit of ill-feeling. And also my in-laws are making remarks already that they'll never get to see the baby as its always with its other grandparents! not exactly the smiling family portrait that it appeared to be!