You and your mother are both very compassionate.
Who handles the finances for your great-aunt and her daughter? If that could be turned over to your mother she could limit her cousin's spending and cut down on incoming hoard items. She could also use some of that for a regular cleaning service or needed repairs.
I wouldn't write an anonymous letter to any doctors. I would make an appointment to talk in person. They won't be able to break confidentiality but when you explain the day to day living situations perhaps they could advise you or would be willing in a legal matter to recommend that you and/or your mother be granted guardianship. Your cousin presents a danger to herself in having grand mal seizures in a hoarded area.
Hoards and the problems that go along with them are very overwhelming. It's a relief to be able to talk about that, even if we can't control it very much, with others who understand. Come and post anytime. There is also a chat. Click the red box Online Support Group to the right.
take care, Minervia