PTSD can be a living hell to the person that has it. Early treatment is best but treatment at any time is essential. I don't know how bad your sister's PTSD is but even so called mild cases are hell. I have nothing but respect for anyone that has it or has had it. Anyone that has not had it can not possibly imagine in their wildest dreams what kind of hell it is.
You can find some good info on the web as well as support forums by surfing around. "Combat related PTSD" sites often have some very good information. PTSD is PTSD but the combat related PTSD sites seem to get more money and seem to have better information.
Meds can help with the PTSD symptoms but treatment is needed to help lessen the symptoms in the long run.
Try and keep your sister away from the Benzodiazepines (Valium, Ativan, Lorazepam, etc.) they are addictive.
A good med to try is Buspar it's in a class by itself. It's not addictive and hard to abuse.
SSRI's like prozac, effexor, etc. work for some people.
For others a tricyclic antidepressant (TCA) might be tried in a dose either for treating depression or as a sleep aid (PTSD folk tend to have problems getting a good nights sleep.) If a TCA is used for a sleep aid the dose will be a lot smaller than if it's used for depression. A good doc will know what kind of TCA to prescribe for this.
Wellbutrin works for some people, it's an antidepressant in a class by itself (it's also the same thing as Zyban so if your sister smokes this might help her quit.)
Your sister should never use alcohol to self medicate for obvious reasons.
What your sister needs is a lot of gentle caring and understanding. She'll have triggers that bring back the PTSD and it's important that you don't trigger her accidently. There is a bunch of physical and emotional symptoms that are hell to deal with. Your sister may have a few or a lot. Respect her hypervigillance if she has any and try to keep things calm. Don't tell her to calm down, be calm and she'll follow your lead.
Here are some decent sites:
http://www.dr-bob.org/tips/ptsd.html
http://forums.military.com/eve/ubb.x...016/m/49910115
http://www.ncptsd.va.gov/
http://www.nimh.nih.gov/publicat/reliving.cfm