PLEASE do not blame yourself!! A lot of times doctors only give you the worst case scenerio. I told you how my mom had the same exact thing happen to her. She broke 2 bones in 3 places, dislocated her ankle, and had to have a rod and screws put in, and she was almost 50 at the time. She gets pain in her ankle now and then, but otherwise she is just fine and walks fine. My husband, 7 years ago fell asleep at the wheel at 1 in the afternoon, and crushed his ankle in the accident. He actually shattered his calcaneus, and it came out the side of his foot. He needed immediate surgery to have a rod and pins put in as well. The inside of that heel bone died and colapsed as a result, but even with that, he only has a slight limp that is only noticable if you watch for it (although his one ankle is notably larger than the other).
Your sister is young, and the younger you are when something like this happens, the better. She will heal quickly and she will be back to walking soon. It took my mom nearly 3 months before she could walk, and just under than with my husband (who is far more determined than my mom). We always joke that I better not ever hurt one of my feet or legs, because I am the only one with a good set
She will have to do physical therapy, and really take it easy during her pregnancy as she will be harboring a little extra weight on that freshly healed bone, but rest assured, she will walk again.
I was on the phone with my mom as she was getting out of her car and slipped on the ice. I beat myself up over it for awhile because if she wouldn't have been on the phone with me, she would have been able to catch herself. You cannot blame yourself though.
Hang in there, pray for your sis, and forgive yourself, you did nothing wrong. Just be there to help her and support her. She will do great.