she is not your responsibility because she is ill.
what about all the people who have to live on PIP and have no one to sponge off; they have to manage.
the welfare system does not expect you to subsidise an adult child, even if ill.
if she really cannot manage she could approach social services. they have workers to advise and support the chronically ill.
and there are also charities and self-help groups.
it really does not help her to go on treating her as if she is a minor child.
it is a difficult situation, but you must not allow yourself to continue to be exploited.
try ringing SilverLine.
and read up on assertiveness, confidence building etc.