Update:
Well I've just spoken to one of the pregnant ones from our uni mates - she actually spoke to the groom last night. The spots only started apeparing yesterday which means the nephew is still very contagious.
We're both going to contact him with our concerns tonight and tell him outright we don't think child should be there - I think she's wondering about not coming, even though she's already had CP.
What's worse, and something I didn't know, groom's father is a doctor himself and really should know this and have told his daughter not to bring the child.
And I really don't fancy getting shingles, no matter how small the risk. I also don't fancy potentially being the carrier of the pox back to my office!
He was only going through the uni mates group to find people - I'm guessing the wider circle of guests will know nothing.