There is fluid in the sinus of an 8 month old baby. she breaths through the mouth?

Clarify question: Do you mean that she is congested, or that she had a study ( CT scan or MRI ) that showed " fluid in the sinus" (infants really don't have much in the way of sinuses anyway)? If congested for less than ten days, it is most likely just a cold, and there is nothing to do other than saline & suction nose if difficulty feeding ( too much can irritate). If other symptoms, she should see her doctor.