D., I had a breach baby 11 months ago, born eventually via C-Section. Here is what I tried:
I was working with a midwife/Naturopath. She tried turning the baby manually around 35 weeks, which for some reason I was instincly against. She used much less force and more time than a normal doctor would have and it was very hard on me. It hurt physically, but mostly emotionally. I felt that if my baby hadn't turned there was a reason. I feel that this is what are bodies are made for...if the system breaks down, I feel there is a reason.
We tried the turn and the baby moved slightly but literally placed it's foot against my pelvis and braced itself. So, we stopped on my direction and on the advice of the midwife.
I also tried hypnotherapy which I heard could work. I had planned to do more, but my water broke shortly after that appointment, at 37 weeks.
My baby was butt first as well, which I'm told is better than feet first since the butt is shaped more like the head and can help open the birth path. Since it was my first child, my Naturopath said it would be very risky to birth the baby naturally so we opted for a C-Section. All went very well and the hospital staff was very supportive of getting me right back to the room for skin to skin contact and nursing. They delayed all the typical procedures of weight, wash, etc. until I was ready (which was the next day or so). If they understand your wishes clearly and know it's important to you, they will often do as you wish. And, if for some reason the nurse you have isn't supportive of your wishes, appoint an advocate to get another nurse in there who is.
I have heard of several people who's baby's were manually turned and all went fine, but I've also heard of those that turned and then turned back.
In my case, my gut feeling was there for a reason. There was a physical reason my baby had not turned. His skull was fused and he probably wouldn't have made it out the birth canal, since his head would not have been able to mold. I would have ended up in an emergency situation.
Bottom line: the mom should trust her gut and her baby. If she feels wrong about the manually turning and feels her baby would not want that, then don't do it. If she feels ok with it, I see no harm in trying.
Best of luck to your sister. Tell her to have a wonderful birth experience. That baby will be born either way and she will be holding a beautiful being in her arms! And, that is what really matters in the end.
P.S. If your sister doesn't have the birth experience she dreamt of, it can be difficult, so help her process her feelings after the birth. I went through a greiving process of not having the home water birth I had planned. All in all, it all worked out perfectly though!