D.C.
Most likely, you and your daughter are sick with the same virus. So the antibodies in your breastmilk should help protect her regardless of whether she is exposed to the virus from either you or your older daughter.
However, breastmilk is no guarantee, and your best bet is washing hands alot, expecially when you sneeze or cough (and sneeze and cough into your elbow, not your hand, if you can't wash your hands immediately after you cough/sneeze).
Hope she stays well!