One of my closest friends has a sister who is 18 and 6 months pregnant. This girl is not capable of taking care of this child nor does she really want to. It has been brought up that she should sign over her parental rights to me. How does this work, will she be able to come back later and take this child from me?

She can't sign her rights away unless you adopt the baby.
She can't take the baby back if you adopt.

You could go for legal guardian but I think she could possibly fight to take the baby back with that.

If she and the babies father, both wish to sign over their rights, you can go to court and apply for custody of the baby,
At this point if the mother does not wish to keep the baby, the father would have the right to have the baby.