I am so sorry that this is happening to your sister.
I was diagnosed at 28.5 weeks and delivered at 29.5 weeks and my son (now 3.5 yrs old) and I are both healthy today.
Everyone's experience with preeclampsia is different, some people deliver many weeks after diagnosis while others deliver hours or days after diagnosis.
Even though the diagnosis this early is discouraging, it's good that she is going to a hospital that can care for her baby if she does deliver soon.
I'm sure she's already received this, but make sure your sister gets a course of steroids to help her baby's lungs develop in case she does deliver soon.
I wish I could help you more, please keep us posted, and if you have any questions about your sister's care, please post them here. I'll be thinking of you and your sister and hoping for the best, Lauren