I'm not sure what you mean by "fluid in the breast" ..... if you mean a cyst; and these do cause pain, it should definitely be ck.d out. In any event she should see her Dr. for a clinical breast exam and any other appropriate testing such as Ultrasound and/or Mammogram. If this is some type of infection then it's even more important to see the Dr. ...... please encourage her to do so. Regards .....
In my opinion your sister in law should see her doctor to better evaluate what is causing the discharge and the pain.Cancer is almost never associated with pain... her discomfort could be hormonal or something more serious.Seeing a doctor,sooner rather than later,is the best thing to do when something is not right concerning the breasts.