M.R.
I don't know, I think you did the right thing. You probably, in your heart, know that you cannot support another child both financially and emotionally. She made the decision to be sexually active and as a result became pregnant which means she now has to grow up and be a big girl and take responsibility for the choices she made, which include financially and emotionally supporting her child. She has made the choice to keep her baby - you weren't telling her she had to get an abortion; you told her that if she made the decision to keep the baby that you were unable to support it.
Her dad and her step mom are saying this is all your fault because they are the ones who are now providing her the free ride and they're resenting it. Her issue has now become their issue, too and their life, free of your daughter's impending pregnancy up until now, is something they have to deal with.
Continue to tell her that you love her as you've been doing. But, remember that your two other children need you, too. I don't think you've done anything wrong. *She* is the one who made the decision to be sexually active and put herself at risk for unmarried, teenage pregnancy. *She* is on the one who made the decision to keep the baby, knowing that you would not be able to support another child in the house. *She* is making choices that have benefits and consequences, so she is the one who will have to live with them.