Goodness me. What a thread!
OP, I am sorry to read how this has gone and I hope you're not feeling too upset.
I am a midwife and I would like to make a few points.
There are many many women that do not eat, are underweight, have hyperemesis (severe morning sickness sometimes throughout the entirety of the pregnancy) and of course those women in developing countries who eat barely anything throughout their pregnancies but they deliver babies, albeit small, everyday.
Food intake is important and we advocate and encourage women to adopt healthy eating options. However, it's a woman's choice. Her choice. And many woman choose to eat unhealthy, smoke, drink and take drugs. As health care professionals, we can advise and make the best plan of care depending on their situation.
This sounds like a placental issue and the regular growth scans, liquor volume and dopplers (checking the function of the blood going through to the baby) will give a good indication on the plan of care for your sister. If the baby is growing then she continues to be managed conservatively (i.e. We do nothing but watch and wait) and if the baby does not thrive in utero then she will be induced early or be given a section so that he baby can be fed in special care.
Some of the information you have provided is slightly incorrect; for example, a baby doesn't dictate that premature labour will stop because it's too small to 'help push down'. If it happens then it happens. Although we can give drugs to try and stop it but it doesn't always work.
Unfortunately, with all of your concerns for your niece or nephew in regards to your sisters choices there really isn't anything you can do. Even speaking to the midwife of consultant isn't going to change things and as many others have said, the woman's rights come before the unborn child's as he/she has none while in utero. Plus I don't think it's entirely relevant if it's a placental issue anyway.
Good luck and i hope all goes well with the rest of her pregnancy. It's a good thing that she's being monitored so closely. 