Artchic, sorry to hear of your situation . I agree with the ideas mentioned and perhaps if you could draw up a Rota specifying what jobs,your,doing when. That way it would be there for all to see exactly what you were contributing.
I find it's easy for people to forget household chores when there is something more pressing to deal with.But I really think you are well within your rights to say. I Will do these jobs but I draw the line at cleaning up after my brother.
All the best, hope it gets sorted.