It's rare these days because there's a vaccine. Unless OP's daughter has a compromised immune system, there's no reason she shouldn't have been vaccinated against chicken pox. So either OP's sister was bullying a small child who not only had chickenpox but also probably suffers from other major life threatening issues on a regular basis. Or, OP is an anti-vaxxer who put her own child through hell and agony over utter nonsense (and is risking the lives of other children who actually have compromised immune systems).

Depending on which is true, one of these two women are definitely an extra shitty person than the original story made it seem. Just unclear as to which is which.