I hope that you are still accessing professional support to help you through this extremely challenging period in your life and that you are also able to proactively put in lots of practical self care to keep you balanced, emotionally and physically resourced, self regulated and coping - in acknowledgment that this is what you need right now in this highly irregular and unusual situation that you are in right now.
This dysfunctional family and their disordered daughter are currently wreaking havoc and distress on all your lives.
It’s all very enmeshed and emotionally charged - because that’s what these toxic people do - spread their drama through a series of people, goading to trigger conflict so that they can divide and conquer and gain more control in the fallout.
To this insecure / unstable girl, you (as his mother) are a huge imagined threat to her deep dysfunctional attachment with your son and she is compelled to agitate to alienate and annihilate you.
Her method is to harass you enough so that under extreme stress you flip out at your DS and prove her right and he goes running to her. This is a text book abuser isolation tactic. Know the game. Don’t take the bait.
The answer lies in putting on your own oxygen mask first, because you need huge emotional resources and defences when dealing with these high conflict personalities types and to then focus solely on protecting your family whilst
knowing that this is temporary and will end.
Detach from her. Block calmly if you need to.
Take her power away - which is to whip you all up into anger and distress - by detaching, anticipating and not reacting to or engaging with her antics. Of course you will feel rage when she pulls another stunt or when your DS kicks off - but express and process that out of sight of them both and only then when you have stopped reverberating calmly decide on your action - ignoring her or a grey rock non committal response if that is less risk - and calm assertive boundaried words, consequences and actions to your DS when he behaves badly.
Ignore as much as you can, be indifferent to the nonsense and very calmly be assertive (not blowing up / letting rip as he experiences enough heightened emotions with her already) and boundaried / stern with your son when needed. Show him how to communicate calmly, respectfully and constructively by de-escalating during flash points - lowering the tone and tension so that you are heard.
The way through is by being calm and boundaried with your own DS. Focus entirely on him. Do not get distracted by her antics and do not get involved with her parents. They are not emotionally healthy and are self serving. Anything you do with them will back fire.
I do think it is dissolving but that may not be a straight line - but try to have faith that you will all come through this. I hope that he had a good time with his friends - that’s a huge step forward in the right direction. Take strength from that. Take care of yourself first.