Loosing a close family member is very tragic and difficult to handle. Your description of irritability alone does not suffice to call it depression. Answering your last two questions first, symptoms of depression are persistent sadness of mood, low energy levels, alteration in sleep and eating patterns, crying spells, negative thinking, reduced interest in previously pleasurable activities amongst others. With age, the negative features are more severe and irritability can be a part. And yes, persons suffering from depression may be missed by significant others especially when mild to moderate symptoms are there. Depression occurs due to certain biochemical changes in the brain which may be precipitated by external stressors or internal triggers, one's personality, social support, genetic makeup, physical ailments, coping skills also have an important role to play.
In your case, kindly take your aunt to a mental health professional, as every individual is different, a proper diagnosis and management can only be done after a detailed assessment.
Irritability can also be simply due to medical causes like anemia,
thyroid disorders, hypertension etc. Or psychological causes like stress. Insomnia etc. So that she will require a thorough body check up. In the meanwhile, Surround her with love and affection, ensure healthy diet and sleeping habits and ask her to meditate and do some pranayaam. I hope this helps.