The easiest way to show someone who is suffering from a mood related issue that it is clinically significant is through something called the cognitive triad. This method was developed by Dr Aaron Beck, a key figure in research on depression, and is simple and nearly 100% accurate.
There are three questions to the triad.
1) Are you feeling hopeless, or can others see that you are acting hopeless about the future? (EX: Plans are avoided or put off. Things seem to not be worth thinking about regarding upcoming events. Holidays that were once looked forward to are not, etc.)
2) Are there feelings of hopelessness about the outside world? (Has interest in outside activities diminished? Favorite things are avoided or ignored? Expressions of despondency or irritability about situations in the outside world [such as work] are shown, etc.)
3) Are there feelings or actions that support hopelessness about one's own self? Has self-care or dress or diet or person hygiene or simply trying to look as good as before, diminished? etc.
If the person has all three, this is a direct marker for a clinical level of depression, whether caused by biochemistry, circumstances or both. The triad does not give a specific mood disorder diagnosis of course, but it does identify clinical issues.
Trauma can cause depression as well as biochemistry, and she sounds like she has been through a great deal. Medications for depression only help about 50% or less of people who take them, and the best practice protocol for depression, mild or severe is medications combined with weekly cognitive behavioral therapy. CBT as it is called, combined with medications where appropriate, can raise treatment outcomes to 80% or more. Most therapist practice CBT; it is short term; highly effective and can be accessed via a referral from a family doctor. As with all therapy, it is confidential.
What would I do if this was my sister? I would ask your sister the questions outlined above. If she has now, or in the past met the criteria I would express my concern to her as well as raising the fact that medications alone are almost never the answer, and that many people recover through talk based therapy.
Her ongoing irritability is also a classic sign of depression, and she is suffering, without doubt. Importantly, not all depressions are severe and one of them, dysthymia (also known as chronic depressive disorder) is very common in 30-50 year old women, yet is it highly treatable. She can easily feel better by dealing with it if she wishes.
You cannot make her go to therapy, but you can be a resource of encouragement for her to do so. Soft words, over time, break hard bones. Encourage her that no matter the reason, she can feel better... and why suffer? Over time, with steady encouragement and motivation from you she has a very god chance of trying some supportive professional assistance. Steven