IF she feels comfortable with a home birth, then yes, they are significantly less expensive. Whether she is medically considered "high risk" will vary from state to state. Different states have different rules for home birth midwives. I have delivered all three of my daughters with midwives in Colorado, and our births were the most wonderful experiences of my life. I was safe, supported and encouraged the whole time. Midwives are highly trained, reasonably priced, and an excellent alternative to traditional doctors.
My best recommendation is this:
First, she needs to decide if she feels comfortable with a home birth. She should probably do some research, read some of the studies that have come out. The idea that we should be in the hospital delivering babies "just in case" is an American medicalized idea. For healthy mothers and babies, the medical evidence DOES NOT support the idea that it is safer to be in a hospital. Nearly every study that has come out in the past 40 years comparing home and hospital births in healthy mothers and babies have shown that home birth is equally safe for babies and MUCH safer for mothers. That being said, if you sister does not feel safe delivering at home, she should not seek a home birth. If she absolutely cannot qualify for any assistance and cannot find a doctor to do prenatal care and deliver her baby, maybe she could pay a midwife for her prenatal care and then go to the hospital to deliver the baby. That would not be ideal, as she would not know the person delivering her baby, and that doctor would not know her history. She would also still have to pay the hospital something. Still, it might work out. She can probably make payment arrangement with the hospital. They will typically cut a portion of the bill off for someone without insurance.
Second, if she wants to seek a home birth, she needs to meet with a local midwife and talk to her. Find out what her restrictions are. She needs to know if home birth is even a possibility for her. As I said before, different states have different parameters for who midwives can attend at home. Just because a doctor calls her "high risk" doesn't mean that she is medically/legally considered high risk. Often that is just the doctor's idea or label. A midwife also might be able to recommend some dietary changes and some alternative treatments for stabilizing her hormone levels. A midwife will certainly offer dietary and lifestyle advice to help your sister prepare her body for another pregnancy. Most midwives will meet with you for at least an hour at no charge. In fact, most doctors will also offer a consultation for no charge (but not for an hour - my doc offered a 15 minute visit for free).
As far as the previous poster's comment to go to the hospital just in case something goes wrong, yes, things can go wrong at home. Sometimes those things could have been prevented if the birth had taken place in a hospital. However, it is not true that the hospital is a risk-free option. The national cesarean rate is up to 31.8%. There are hospitals where it is up to 40 and even 50%. Women in hospitals are subjected to unnecessary inductions, pitocin drips, episiotomies, and cesareans. Babies are routinely removed from their mothers in the important moments after birth. Breast feeding relationships are routinely interrupted by "standard procedures." It is sad, but the fact is that babies die in hospitals also. Babies in hospitals are also are born injured and brain damaged. When you choose a hospital birth, you are not guaranteeing your baby a safe birth. Many, many women do not choose home birth because of the "experience," but rather because of the dangers of a hospital birth. Your sister needs to weigh her options and make the right decision for her and her family without buying into the horror stories.
If your sister would like more information about home birth, or would like to hear about my experiences, please feel free to private email me. I would strongly recommend that she reads Henci Goer's "The Thinking Woman's Guide to a Better Birth" no matter where she plans to deliver. I can also recommend some other wonderful books if she wants to do more reading about birth. I love to talk about my births, and I have extensive contacts in the Northern Colorado area among the birth community.
Best of luck to your sister,