It probably won't prevent her from getting it, but I can promise you it would be worse if she were not nursing. I would also recommend giving your older daughter some of your milk (via sippy cup if she's not interested in nursing). It'll help keep her hydrated, etc. And I would advise against any probiotics for the baby. ANYTHING besides breastmilk that goes into a baby affects their gut and thus their ability to absorb nutrients and increases their risk of allergies. And the more that goes in, the higher the risk. It's just not solid foods that fall under the 6 mos rule when it comes to allergies and gut issues. So I would avoid anything if not absolutely necessary and I don't imagine that being the case - breastmilk is truly great!