Was going to say the same as Tinks, as frustrating as it is, I'm sure your SIL didnt mean to put your DD in harms way. Fingers crossed she doesnt get them, but maybe stock up now on things that will help comfort her if she does break out in the middle of the night, like lotions etc. you mentioned that you have never had them but are you vacc against them? If not maybe talk to your Dr about getting it done (not sure though if you could while you are breastfeeding, wont hurt to ask though).

Really hope it stays away from your whole family.

you