I want to adopt my sister's unborn child
Are you sure that's the best option for everyone?
I will adopt this child because I want to make a child happy.
It is good to adopt your siblings child if they don't want them. You should make sure she wants to not keep the child before suggesting it.
I know she wants to get rid of the child.
Then that's a nice thing to do. Make sure you have a lawyer and do things properly so that there is no complications in the child's future.