My info is that babies get a small amount of protection via breastmilk in
first few mo. if mom has had ch pox.  However, the exposure to a sibling in
the home is usually pretty potent, more potent than the casual exposure
that the older sibling gets at school and then brings home. So it always
seems as if the second case in the family is much worse.  Last winter I had
a six week old bot fed baby with full blown ch pox from sibling contact.
Sincerely, Pat in SNJ