OJ, I can't can't believe the level that they are willing to stoop to all in the name of what's best for Steve. If they really cared for Steve, they wouldn't hurl such abuse at you, his wife who loves him and is caring for him.
Firstly, please see if you can have a friend with you as much as possible. Not only for your own emotional support but also as a witnessed as to what is said.
Secondly, talk to the Mac nurse but also approach the carer agency and see if you can't get some sort of written conditions which Steve's relatives have to abide by when they are in your house. Also could you not run it by the hospice as well. They might be able to get you legal advice on how to make such a document legally binding.
I appreciate that you don't want to stop Steve's relatives from seeing him, but things can't go on as they are. What a horrible spiteful thing to say about understanding why your parents gave you up. That is emotional abuse and you shouldn't have to put up with it, now of all times. You'll need all your physical and emotional strength for what is ahead.
((((((hug))))). I am here for you anytime you have a free moment and feel like talking. xxx