Re: Is it depression?
Frequent irritability, anger, and agitation are common symptoms of depression, but they are also symptoms of other disorders. Withdrawal from relationships is also a symptom of depression.
However, only a professional can properly diagnose depression or any other disorder.
As stated many times on this forum, to get help, a person has to admit to themselves there's a problem. Having dealt with family and friends in constant denial, I know it is no easy task to convince some people that a problem exists and it's time to seek assistance.
Perhaps you can convince your sister to get a general check-up from a family doctor, and go from there. It's a start.