I'm sorry you and dh are faced with this situation.
If your sister was in your home crying about it, I assume you are close and she knows what you have been through. Hopefully, she is mature enough to realize she is putting you in such an emotionally difficult situation.
I think it's ok for you to be mad and hurt. It's circumstances like this just don't make any sense. How can a couple that want babies so badly be denied, and then a young unprepared person is given a gift they don't even want? But ultimately, she is your sister and you will love that little baby, and you'll be an awsome aunt!
Try to be the better person.