Hi, @shk38, the parents of your husband's nephews and nieces have made a very silly and totally scientifically unjustified decision. However, as long as your baby can be safely vaccinated, there should be no need to keep her or him away from the cousins under most circumstances. Initially your baby will have immunity from you, and as that wears off, the baby's routine vaccinations will be appropriately scheduled to take over.
Well done on getting your baby vaccinated -- you are definitely doing the right thing (and the thing that ALL the evidence points to being safest for your child as well as for the community).
Obviously if any of the cousins is unwell in any way, you should keep your newborn well away -- this would apply whether or not they were vaccinated.
The cousins parents are, frankly, relying on other people to keep their kids safe from diseases that can kill. I don't have any problem disrespecting that kind of decision. How you react is entirely up to you...
If, on the other hand, your child is unable to be vaccinated for some reason, then it would not be entirely safe to visit with the cousins during any outbreak, or otherwise, at any time. Also, many vaccines depend on 'herd immunity' -- the added protection from others around you also being vaccinated, so that you are unlikely ever to encounter the disease even if your immunity is not complete. Herd immunity is how small pox was totally eradicated, and polio mostly eradicated. These are diseases that used to kill or cripple children worldwide.
Most vaccines are a little less effective on individual level and depend on others around you also being vaccinated. Mumps is one. So even if your child is vaccinated if there is a major mumps outbreak your child could be affected. (I know this because my daughter was vaccinated and she got mumps anyway -- would not have happened if other parents had also been responsible...)
Measles kills children. Your in-laws are counting on people like you, who vaccinate their children, to keep their children safe -- so that even if they are not immune, they won't encounter the virus. If too many people rely on others to have the vaccines, the percentage falls, the herd immunity fails, and children once again get unnecessary childhood illnesses. Measles can kill. Mumps can cause disabilities. Rubella is almost always nothing to the child who gets it, but can be devastating to an unborn child of a mother who encounters it.
And it is all totally unnecessary.