You aren't horrible. You didn't tell her off, you were able to be supportive...
.. and when you hung up, you were able to let those sad feelings out.
I've been in this situation more than once with my own sisters (me, multiple miscarriages, sisters seem to get pregnant if someone breathes on them). Personally, I am not sure anyone is going to judge you, and if they are, that's really their problem. There's nothing wrong with wanting a second child-- it doesn't mean that you don't deeply love and cherish your own kid, it just means that you envision another face at the family table. That's okay. It's really okay to want that, and just to feel disappointed that it wasn't you announcing the pregnancy this time.
Please don't beat yourself up over this, try not to let this come between you and your sister. I'm sure she is pulling for you as well. It's okay not to share this aspect of your feelings with her, and it's okay to have the feelings you do have. Coming here to vent was much healthier than stuffing those feelings down or holding them in. You aren't a bad person to have these feelings. It's pretty common.