Sorry to hear about your sister. Symptoms such as sadness of mood,  irritability, disturbed sleep, weight gain, crying spells, feeling helpless, lonely, hopeless , demotivated are all signs that she is depressed since quite some time. Since she is also suffering from hypothyroidism her symptoms of depression will worsen. It is affecting her overall life, her personal life is stalled, she is unemployed. As the functionality is affected it should be of concern and psychiatric help should be taken at the earliest. Depression is a result of neurotransmittor imbalance in the brain resulting due to environmental or bodily stressor in a geneticy vulnerable individual. There is treatment available to set the same correct, dont let her suffer too long and waste her precious life productivity. If the efficiency is 100 percent there is no sense is living life at 50 percent efficiency because of an invisible disability.

I appreciate the concern you have about your sister. Do encourage her to meet a counsellor and share all her feelings from her perspective and ease her out. She will and can definetely get back her life and lead a happy and successful life.