Sounds like the younger sister got jealous of her sister's new toy. She then baby trapped you to ensure she steals you from her sister.

I would tell her that she should get rid of it. You will pay for everything and take her to the appointments. That if she doesn't she will lose her sister and you as well. Tell her that the situation between you, her, and her sister is already over and ended the night you slept with her. You will not be around for the child and men will not want to date a single mother especially one that became a mother at 20. Her next 18 years will be a struggle through poverty and raising the child alone.

These aren't nice things to say but this is how you get out of a baby trap.