It is quite understandable that like other illnesses the best and accurate diagnosis in a psychiatric disorder too can be made by assessing the person concerned as well as having longitudinal behavioral information from significant others. There can be numerous reasons for a particular symptom and the best understanding is got from the subjective viewpoint of the person concerned, viewed by experienced lenses.
As far as the diagnosis of Schizophrenia goes, although some of the symptoms described by you can occur, but your information lacks the core symptoms/key symptoms on which the diagnosis of Schizophrenia usually rests.
As far as the symtomatology mentioned, it can fall into the category of Borderline Personality disorder. However, we need to understand here that there is usually no specific medication, which can treat a particular personality disorder. However, medications may be used for symptomatic improvement. And once the symptoms are under control long term psychotherapy can be and has been found to effective, however it needs to be done by an experienced/ psychologist/ therapist.
All medications can have side effects and long term use of certain medications more so. But, it is the balance between the symptomatic improvement and the side effects, which decides that a particular medication can be continued or not.
Usually, aggression and unstable moods can have an antecedent. However, if they are quite frequent sometimes medications other wise known as mood stabilisers (like Valproate) can be helpful, however, they need to be given in adequate dosages and prescribed and monitored by a psychiatrist. Antipsychotics like Risperidone usually provide short term management strategies in aggressive patients if the aggression is not secondary to psychosis.
There can be major psychosocial impact of psychiatric illness like job losses, divorce etc, which can infact perpetuate a vicious cycle, by itself.
While it is difficult to have a non critical approach towards the person- that is the key in encouraging the person go to a mental health professional like a psychiatrist, clinical psychologist. While the medications continue, I would firmly encourage you to use all the positive resources at your end to help your sister get herself assessed by a psychiatrist and then carry on further psychotherapy and medicines if advised.