First of all, it's great that you're concerned and willing to help. Often, family and friends are at a loss about what to do in a situation like this. Luckily, there are things you and your sister can do to help her get proper care. Next, get in touch with a local community hospital and ask about childbirth preparation classes and obstetric services. By law, hospitals are required to provide a certain amount of charity care.
Many hospitals offer childbirth classes at little or no cost.
Just being around other expectant mothers may reassure her that she's not alone and help her gain some confidence. I'd also suggest that your sister apply for Medicaid if there's a chance she might be eligible. This is the combination federal and state health insurance program designed for low-income people. It's far better to apply and be rejected than not to apply and miss out on this program. Many of these centers are operated by anti-abortion groups, so before you seek help from one of these centers, find out who operates the center, whether it's licensed by health authorities, who provides medical care, and what the center's policy is on unwed mothers.