Thanks for using the forum. I am happy to address your questions, and my answer will be based on the information you provided here. Please make sure you recognize that this forum is for educational purposes only, and it does not substitute for a formal office visit with your doctor.
Without being able to take a history and examine your sister, I cannot tell you if she's had Guillain-Barre Syndrome (GBS) previously. While GBS can recur, it usually doesn't. Your sister's apparent history of having this 3 times is very unusual but not impossible. Her doctor telling her she's at increased risk for having infections while pregnant is true. And often GBS occurs sometime after an infection. That said, there's no way to know (at present) that any one infection is going to cause GBS and we all have many small infections (e.g. of our skin, common colds, etc) frequently anyway.
A recent study (Acta Obstetricia et Gynecologica Scandinavica, Volume 83, Issue 4, Pages 319-325, Published Online: 8 Mar 2004) of 30 cases of GBS during pregnancy resulted in abortion in only 3 of the 30 cases (which, in retrospect, didn't speed the recover of the mothers and accordingly is NOT an indication for abortion). Intravenous Immunoglobulin (IVIG) has been successfully and safely used in pregnant women for more than 25 years including 22 of the above 30 patients. Only 1 of the 30 women died and 1 baby apparently developed GBS when it was born (the only known case ever) thought the baby was able to be treated and recovered. Despite neurological deficits, uterine contraction was not effected and the ability of normal vaginal delivery was possible. So GBS by itself is not an reason for c-section.
I would recommend printing what I've written above with the journal article reference and bringing it to your Ob/Gyn doctor. I hope this is helpful.
Your sister needs to see some specialists before she makes any decisions about the pregnancy. First on her list should be a perinatologist appt (specialises in high risk pregnancy), also she could consider seeing a rheumatologist who specialises in immune disorders and will probably be able to help her know what the risks of pregnancy are for her and the baby and/or a neurologist to see how she is doing right now. Potentially a consult with a reproductive endocrinologist may be a possibility too, but she will need to tell anyone she sees, how far along she is, so an appt is made in a timely way. Most of these specialities will be listed in the phone book under physicians.
The issue with being pregnant with a disorder like GB is that the pregnant body has to adjust and realise that the fetus is not foreign. If the female's immune system thinks that the pg is foreign then it can work to attack the pregnancy and result in a miscarriage.
Depending on how far along in the pregnancy she is, she probably has a few wks to make these appts and make her decision. I suspect the Dr is correct in saying it can send your sister back into another GB espisode, but am not sure about the risk to the baby.
Wishing you peace of heart while you all go about figuring the best decision possible. Warm hugs, this has to be very hard for you all.
Fionajnz