You may have her talk to her doctor about it. I know I was in a depressed state for several months before I finally broke down and let the doctor put me on Paxil. I would get very stressed out doing everyday things and my kids would get on my nerves within minutes of them coming home. I would also cry at the drop of a hat.
The paxil has really helped, but having someone to talk to about it all also helps. Maybe you could help your sister find a counselor and go with her the first time. You have to remember that her life spun way out of her control not too long ago. She may feel angry or confused and you are a "safe" person for her to let down her guard with. Give her some space and time, but I wouldn't cut the trip short.