Personally, you have to do what is right for you, and that may be very different than what is right for me or anyone else on this board.
Here are the facts:
It sounds like 2 of the family members are NPD and BPD. They each have a partner, and their two partners are both enabling of the other.
Since you are NC with SIL, I'm going to assume there is "history".
We are in a Covid pandemic.
Point of clarification: are all 4 of them wanting to come at once? And maybe bring kids too? How old are the "kids"?
Are you comfortable hosting that many people who have travelled from out of town in a pandemic?
Where I live, we have been told to avoid all non-essential travel. People are largely observing this. Personally my H and I wouldn't be comfortable hosting that many people for any length of time at this time. Covid is blowing up where I live.
Is it possible the BPD/NPD is a secondary risk to the physical Covid risk? Which is the biggest risk?
Another question: are you the "destination"? How long would they be staying? Is it a single night enroute to another destination, or are they wanting to come and stay for a visit?
Lastly, to answer your other questions, if two couples are coming together, and one person from each couple is either NPD or BPD, the fact that they are coming together could mean that this could indeed be setting up possibility for triangulation. You know them and have experience with them, so you are probably the best person to answer your question.
Radical acceptance to me does
not mean accepting manipulations or being a doormat, or being used as a hotel for accommodation, or a quick get out of town holiday. Perhaps it makes a difference why they are coming? (Is it for medical reasons, or important family business?)
Radical acceptance to me is accepting my mom and her BPD, and having no expectations of her to understand me, or treat me how I wish to be treated as her daughter. Instead, I have been the one to change how I communicate with her, how I manage my relationship with her, and how I react to her when she goes "BPD" on me. I accepted that it was all on me, cs she was never gonna change. "No expectations."
What does your H think about his family and the kids coming? What does he want to do?
Say no and arrange a gathering later and on our terms?
Hmm. When dealing with PD's, it is always helpful to manage relationships on your terms IMHO, even without a pandemic going on.

A lot probably depends on where your H stands on all this.