I see nothing wrong with greeting guests at the door and filling them in on what is going on.
Tell them something like "We are glad they are here to see cousin, she likes visitors and loves to talk about her kids, sports, crafts, memories, etc. However, even though we know we don't need to say this, we need to mention to you not to talk about her illness or mention treatments, that surely more can be done, etc. as these upset her greatly and we know you wouldn't want to upset her. We have a list of ideas for conversation if you would like to look at them."
Then, if they start in, quickly stop them--there are excellent suggestions by previous posters. Even ask for their help in the kitchen to get them out of the room and them remind them.
I also like the idea of restricting who can visit. At this stage, I would limit it to the people Amanda really needs/wants to see.
Hugs to your cousin, her family and you.