It doesn't sound like you see them much anyway .
Just give them time to get through the new parent hyper-protectiveness. Obviously your baby survived having unvaxed siblings, eventually that will sink in.
It could be worse; someone on the main vax board posted awhile ago that their children were banned from the home of their first cousins (twins) for two YEARS because the poster's children were unvaxed.
You could say that you understand as the number of pertussis cases are particularly high this year, but that you hope they know that the vaccine does not prevent transmission of Pertussis so any visitors pose a risk -- though true, it may be viewed as passive-aggressive so it may be better not to say it...
I do hope that once the baby is older (and the parents start doing their vaxes) then they'll stop with the hard line. The kid can't be kept in a bubble its whole life and there will be plenty of people out there, vaccinated or not or somewhat, who have the potential to transmit all sorts of illnesses.