Probably not. If your sister is a baby she can be easily treated by medicines. Thrush is just an overgrowth of yeast in the mouth. There is an over the counter medicine called gentian violet. One prescription is nystatin. In rare cases thrush can suggest an underlying problem or disease such as an immunodeficiency or hiv. Thrush can also come from some medicines such as inhaled corticosteroids taken for asthma.
Depends. Whether oral thrush is something to worry about or not depends on several factors, including age, current medications, other diseases. Your sister should talk to her doctor about causes and treatment.