They sound like a charming bunch. I would suggest, if you have the energy, getting a solicitor to write to them to say that they either behave like adults in YOUR home, else they will be excluded. Which IMO you would be within your rights to do. This is a horrible thing to deal with on top of everything else.
FWIW (and im not sticking up for them) my mum was convinced the hospital were ODing my dad, because she was angry and scared and didnt understand that he was actually better off doped up and not in pain. Also his alzheimers made him pretty much irresponsive too. That is not to excuse their behaviour, but so you don't feel so hurt by it. It sounds to me like they are jealous of you almost, you are in control of things (as you should be) and they probably feel helpless - BUT you are his wife, the woman who steve adores and he wants YOU not them. My Dad only wanted my mum at the end, to the point where he would look straight through me like i wasnt there and respond to my mum (it hurt but i was pleased he had my mum - we were very close, i wondered if it was because he could only bear the person closest to him to see him like that). I am so angry for you OJ, i feel like shaking Steves family - Let them make whatever threats they like, but not in your house, you tell them, they either are civil or they dont come in, end of story.
OJ i hate to ask this, but do you have any idea how long steve has?