Thank you for your query at DoctorSpring.com.
I understand your concern and will do my best to help you.
You can safely go ahead with the visit.
The only way your little one would be affected is if your wife did not have chickenpox earlier and developed an infection now.
Since she has had chickenpox in her childhood, she has lifelong immunity against the Varicella zoster virus(VZV), which causes both chickenpox and its reactivation-shingles.
Reactivation of VZV occurs either in the elderly or immunocompromised host. Even if your wife is exposed to her grandmother now, there is hardly any risk of her getting infected as she is already immune; and practically no risk for your baby as long as he/she is safe inside the womb.
In fact, the fetus of an immune pregnant woman exposed to the varicella zoster virus(VZV), is safe from VZV. It is well documented in the literature that IgG antibodies are transplacentally(through the placenta) passed to the fetus providing the necessary immunity. These antibodies persist in the newborns for up to 6 months of life.
I hope that was helpful.
Feel free to ask queries.