The NHS website has a page about chicken pox at this link. It mentions that the chickenpox virus can spread to other people from 2 days before the spots appear until they have all formed scabs – usually 5 days after the spots appeared. Spots can dry up after 3 or 4 days and can then sometimes be followed by a further crop. But overall the infectious period is from one to two days before the spots appear until they are all dry.
Chicken pox is thought of as being highly infectious so I think to be on the safe side you should give some thought as to how likely it is that anyone else in the family who might have come into contact with your granddaughter could also have or be harbouring it. Before the spots appear, the illness usually starts with one to two days of fever and feeling unwell but his may be absent, particularly in young children. The incubation period is between one and three weeks, this is the time from exposure to the virus and developing disease.
Probably the best thing to do is for your sister to discuss this with her specialist or specialist nurse and see what they have to say.
I hope this has been of some help. Give us a ring if you would like to talk anything over. The number to call is Freephone 0808 800 4040 and the lines are open from 9am till 5pm Monday to Friday.
