I'll keep this on the etiquette side of things, as my primary question was asking for one liners to deflect the parenting.
Obviously this has moved to the "cut direct" side of things, and that is what I'm going to do for a while. This forum has taught me that "But they're FAMMMMIIILLLYY" is NOT an excuse to put up with things. there's a quote: “There are the families that we are born into, and there are the families that we choose our circle of friends.". Well, now is my time to be with my circle of loved ones, and they aren't those who birthed me or who I birthed.
If and when people ask you about the situation, your response will depend on your level of intimacy with the person asking.
Close family members should be aware of the situation fully and never demand that you justify cutting off this child for awhile. This may or may not extend to close friends. If you trust them and they know you well enough, there should never be a question of justification.
For friends and family members who are not in your most intimate circle, they do not need to know details unless you choose otherwise:
"That is a topic I do not wish to discuss. Beandip?"
"She and I are having issues. I do not wish to discuss it. Beandip?"
"She severely disrespected me and left me sleeping at a rest stop. I do not wish to discuss it further. Beandip?"
"The problem is between her and I. I do not wish to discuss it with anyone. Beandip?"
For those who persist in demanding details (and I have had this happen),
"I said I do not wish to discuss it further."
"I cannot believe you are persisting when I said I do not wish to discuss it."
You always have the option of getting up and walking away. If that person is a guest in your home, you can ask them to leave.
It is a private matter that is not a topic for public discussion.