Power of attorney ended upon death so your sister has no authority to do anything with that. Your mother would continue to receive her Social Security or now that her husband has died, she can get whatever was paid to him, assuming they were married for at least 10 years.
Otherwise, your father's benefits would end. I suggest that your mother contact her local GA Social Security office or, as you have internet access, that you go to the Social Security Administration's website.
Is there a possibility that your sister has a power of attorney over your mother? If so, the Social Security checks are still paid to your mother unless your sister is designated as a payee. Your sister, if she had power of attorney over your mother, could manage your mother's bank account.
Regarding VA benefits, I don't practice in this area but again, you would go to the Veteran's Administration website and read the information there or contact them. I don't know what the VA benefits were for - if it was retirement and there was a surviving spouse option, then the benefits would pass to your mother. Otherwise, they would end.
