The parents know she is a head case - she has most likely run rings around them for years, driving them to distraction and probably divorce ..... then your sweet little boy came along and gave them a night off ... and they looooove that so they have done anything and everything to keep him entertaining and soothing her - as it relieves them of their responsibilities.
Your son is now her carer ..... stuck there with the trauma bond she created.
She looks to have BPD traits and YOU are her “target of blame” - google all this stuff and how to handle this situation.
You need to be careful not to put heat under this as you will inadvertently fan the flames.
You are taking this too personally IMHO - your energy seems intense and your anxiety has triggered you into reactive controlling behaviours - which is understandable but not the right way to go. It’s almost like you have lost sight of your own DS in this and you have now found yourself escalating in a pitched battle with a 14 year old girl with MH problems. And if she has BDP you are dealing with a v high suicide / self harm risk. Know that.
You can play your hand much safer and smarter here.
Adopt a strategy of taking responsibility to inform your DS of identifying abusive behaviours, setting boundaries and enforcing consequences.
Teach him how to make this a healthy RS - not drive YOUR intention to end it - because that is where it backfires because that’s not what he wants or is capable of right now. Aim for a different interim goal.
He wants to be in a RS with her - he wants to be in a balanced happy healthy RS with her - inform him how to incrementally set those boundaries - hand him the tools (from professional source) and then walk away and let HIM do it.
Then kill her with kindness - he wants you to get on with her.
Unlock horns for the long game.
Once YOU are out of this drama triangle and he is left to manage her on his own the whole valency will change and the RS will likely collapse in on itself and fizzle out.
Imagine it like a tug of war - he is in the middle but she is fighting you - drop the rope and she will be discombobulated.
Bide your time.
Don’t react to any of their nonsense. Have a long term strategy. Get some professional help and focus on yourself and your other DC. Trust that this will all be over by the end of the year.