I agree with DoctorWhoLuver. The cousin doesn't strike me as someone who is mentally ill. She strikes me as someone who always thinks she is right and everyone else is wrong.
If the fish die, it's because they weren't hardy enough to survive and will be replaced with a fish that are hardy. If you have a bigger set up with more supplies (i.e. heaters, filter, thermometer) than her set up, it's because you are spoiling the fish. If you do more water changes and gravel syphoning/sand cleaning, you are being paranoid and need to stop hovering over your tank. If you tell her that what she is doing is wrong, then you are an arrogant know-it-all trying to backseat drive her pet care.
I don't waste my advice on that type of person. DX