Is there a cost to adopt a sisters baby whom is willing to aign over papers?
Get a lawyer. You have a couple of ways to assert your parental rights.
Are there families willing to adopt 18 year old girls in South Africa?
There might be, but you will have to go to South Africaand ask anyone if they would like to adopt!
Is there a site where you can find parents willing to adopt adults?
Are there any mommies who are willing to admit that another baby is cuter than their own?
No, I think most mommies don't think other babies are "cuter" than their own, but rather "just as cute".
How much does it cost to adopt a baby?
Adopting a baby internationally roughly costs 20-25k. Half of this cost includes transportation to that county.