Sister diagnosed with Mrsa, she has not been in a hospital. How serious is this, and could she have gotten it? Best treatments to cure?

MRSA. if your sister is not in the hospital she unlikely is ill enough from MRSA. It usually can be treated as an outpatient. Bactrian or Clindamycin or Doxycycline are great for it. If she has an abscess it is most important to get it drained by a professional, do not squeeze.
MRSA. MRSA is a bacterial infection resistent to penicillin and many other antibiotics. It is usually seen in hospitalized or other patients such as nursing home and dialysis patients. Another type (community associated MRSA) has occurred among healthy people and is spread by skin to skin contact. Drainage of the infection and use of Antibiotics specific for MRSA such as Vancomycin is the treatment.