I'm sorry to hear your family is so distressed. My name is Aliza and I'm a Medical Librarian. I'm retired, but I help out on this site giving advice (usually non medical), and offering referrals to doctors, hospitals, agencies, etc. and research for people who need it/ask for it.
I hear things don't sound like they're going well for your sister-in-law. I think that perhaps your sister-in-law might benefit from getting a second opinion from another doctor at a major Cancer hospital. There are organizations that help fund people travel to these places often for free or little cost (as cancer patients even including a caretaker [I can try to locate that information for you]).
The best hospitals for Cancer are as follows (all of them are very good, one not better than another): Memorial Sloan Kettering Cancer Center in New York City; Dana Farber in Boston, Massachusetts, Johns Hopkins in Baltimore, Maryland, MD Anderson In Houston, Texas, and University of California San Francisco (UCSF) in San Francisco, California. Another hospital that is good, but not a major cancer center is The Mayo Clinic (which has a branch in Jacksonville, Florida, in case no one in your family can take that much time off to go far away).
I am guessing that your sister-in-law has been seeing a doctor at a local hospital in South Carolina. Sometimes the larger hospitals in the bigger cities have access to more advances in medicine than the smaller ones do, so it is worth a trip to one of these hospitals.
I will see to it that you are referred to flight information for Cancer patients as well as housing information. Sometimes these hospitals also offer housing information for patients and guests as well if the patient is to be examined or treated on an outpatient basis.
I hope what I said might help you and your famlly realize that there may yet be things that can be done to prolong your sister-in-law's life. I cannot say at all with certainty as I'm not a physician and if I were I don't have her medical information. Until you get to one of these places, it's up to your family to make sure that the doctors who are currently treating her make sure that she is comfortable and that she is not coughing all day. I;m fairly certain that there is medication that would help that, even if she needs oxygen. Look into her health insurance to see what benefits she's entitled to for drugs and home health care.
I wish you, your sister-in-law, and your family the best possible outcome in a difficult situation.