Your sister's action is what we refer to as Deliberate Self Harm. This act of hers is to be taken seriously, since it is a cry for help. You must consult a Psychiatrist at the earliest, to get a thorough history and mental status examination for her done.

Your sister will require an urgent consultation by a Psychiatrist. Cutting wrists is a sign of self harm. Such persons are more likely to attempt or complete suicide compared to others. To prevent the situation from worsening please take her to a Psychiatrist immediately.with medication and counseling it is likely that she will get better. Ensure that she is not alone at any time, and all sharp or harmful objects are not within her reach. Keep a watch on her to see that this does not repeat.