What a lucky sister (and Mom for that matter) except of course for the mastitis.
Please note I have included a bunch of links below. They should have all the info she needs. I can't offer an opinion on whether she should renew the antibiotics (if she is currently taking them) but she should at least finish whatever she is taking. I am guessing she is concerned that she and/or the baby will develope Thrush (yeast infection) because of the Antibiotics. Most of the links I have given below are to Kellymom. If Thrush is a concern do a key word search Kellymom using the word "Thrush" and you will learn all she needs to know about preventing and treating thrush. I will give you a brief and incomplete summary: limit sugar intake and junky carbs, change bra/undergarment daily or more often, wash in hot water, bleach or vinegar ...
Regarding mastitis: NURSE, NURSE, NURSE; moist heat (moist heating pad, microwaveable thermal pad wrapped in damp face cloth, hot shower, etc.); gentle massage in combination with moist heat; "combing" downwards with fingertips or a wide-toothed comb in combination with moist heat; rest.
Co-sleep to promote nursing and rest at the same time. Kellymom has safetytips on the "Family Bed" (use that as the search term if co-sleeping doesn't take you there). Oh did I mention: NURSE, NURSE, NURSE
LINKS DIRECTLY BELOW
Plugged Ducts and Mastitis
http://www.kellymom.com/bf/concerns/mom/mastitis.html
Recurrent Mastitis or Plugged Ducts
http://www.kellymom.com/bf/concerns/...-mastitis.html
Engorgement
http://www.kellymom.com/bf/concerns/...gorgement.html
MASTITIS
http://www.askdrsears.com/html/2/T022200.asp
What can I do about my repeated cases of mastitis?
http://www.lalecheleague.org/FAQ/repeatmastitis.html
Natural treatments for nursing moms (NOTE, you’ll need to scroll to the relevant section)
http://www.kellymom.com/herbal/natural-treatments.html
Good luck and God Bless you for being supportive.
~Cath