My sister witnessed a cat being killed and is traumatised. How can I help her?

Trauma from cat. Witnessing a traumatic event can sometimes lead to a more serious problem such as post traumatic stress disorder (ptsd) manifested by nightmares, increase startle response, angry outburst, recurring flasback of the event provoked by some that mimic the renactment of the trauma. There is now new mecication treatment for ptsd. If not treated it can lead to other anxiety conditions.
Therapist. Social and family support is important in a time of trauma. She may also benefit from talking with a therapist or a psychiatrist.