Not vaccinating is not against the law.
However, depending upon your state's seat belt laws, your SIL may very well be breaking the law for not placing her children in appropriate restraints.
How very rich of her to point an accusatory finger at you. And I could see how it could bother you, so vent away, mama!
Meanwhile, I would stop discussing your child's vaccination (or non vaccination
) status. It truly is none of her business.