Aw bugger! That's not good at all! I would say your DD would have a high chance of getting them, there is nothing you could do to prevent it. But maybe get down to the chemest and see what products are availble to help your DD if she does get them, so that you are prepared (pinetarsol and calamine lotion and pain killers are regulars, but not sure on the age with regards to usage)

please don't be angry with your SIL, the most contagious time for chicken Pox is when there are no signs. She wouldn't of known xoxox