Yes, but if the child is allergic to dairy, and if there's a risk of a soy
allergy too, the breastfeeding mother also needs to avoid the allergens as
they will be in the breastmilk if the mother ingests them.

When I was nursing my milk allergic son, I also avoided dairy.  It was a
challenge, but I felt he was worth it.

If your sister is concerned about the allergy and is not finished weaning,
have her also avoid dairy until she meets with the doctor.