Mumsy, you're not the only one it's happened to. I ran a parents support group for several years and I think most members experienced similar.
There are things you can do. I'm a writer so I know the laws surrounding the internet and various breaches.
The owner of any site has a duty of care to enforce any members right to privacy, (are you reading this Mumsnetters?).
If a poster writes a defamatory post and any individual can be identified by it leading it to cause them loss and distress, they can be both sued and the site taken down.
Were you named in the post Mumsy, or has there been a way that you can be idenitified? If so, you have legal rights.
What's written on the internet is the same as what's written down in hard copy letter form. If this is exposed and read by third parties, the same law still applies.
Furthermore, any person who has to openly trash their relatives online in any kind of naming and shaming is only asking for trouble, It not only remains on the site, but if the site owner doesn't remove it, Google keeps it forever.
This means if someone does bring a legal action, and the post disappears, it can be retrieved and both poster and site owner can be facing trouble.
So, the moral of the story is, don't post defamatory comments about your family for the whole world to see and if you feel you must, because that's where your integrity is right now, don't name them