If your sister's blood counts are low because of a damaged liver, it may be harder to fight off the infection. If she is currently on treatment, it may cause problems as well due to low blood counts. If she is currently not on treatment and and blood work is ok, shouldn't be a problem. She will have to remain diligent about fighting the MRSA. My daughter had MRSA and it took 4 rounds of antibiotics to finally clear it up. It is very difficult to clear up and she is extremely healthy.
My ex-sister in law had pneunomia and had a very difficult time fighting it off, she was in a coma and life support for quite some time b/c she let it go and went to the hospital way to late. She is better now, however she did not have mrsa. Is that a separate infection that is not related to the pneunomia? Is it related to an open wound? Hopefully someone will chime in on this, but the more information you give the better the answer you will get. God Bless