So you get to be an Aunty thats great and you will be able to help your sister,and enjoy the Baby look on the positive side now it is what it is and its another little one.She will need you all ,has your Mom accepted the little one aswell?
Everyone has to accept the consequences of the decisions we make, the good ones and the not-so-good. Your sister made a mistake - period, that's it. She is young - and young women (and guys) make mistakes, sometimes big ones (adults, too).
You don't mention your age but I assume you are a young person as well. Try to imagine yourself in your sister's situation, scared and confused. Really try to get this understanding, it will help. And then, go and support your sister and her new baby. She needs your love and understanding right now. She needs you to be her ally. Treat her how you would want to be treated. That's what sisters do for each other.
Try not to judge, we all make mistakes.
Good luck.