I lived a difficult childhood and didn’t feel safe in the family home with a mother, who favoured my bullying brother. A brother, who enjoyed physically hurting me and mentally destroying me. I had a workaholic father, who died when I was this ‘hard to handle’ age and my mother instead of comforting me was cold and vile at times.
I understand that your dd1 had a lot going to process and it doesn’t sound as if she has been able to do so and much as you’ve tried a couple of things, she is probably emotionally stuck at age 16. However, this doesn’t excuse her behaviour.
Due to my experience, I don’t think waiting for NHS therapy is a viable option. I managed to get some NHS psychotherapy with a clinical psychologist age 21 by which time I was in deep, deep depression and had been suffering from depression for many years. Tbh despite going for a year, it barely scratched the surface. I didn’t really become the much wiser me today until I had some amazing therapy in my 40s when I started to tackle my CPTSD. This therapist worked with me for 4 years and used various techniques including hypnotherapy. I have had other therapy since of shorter duration btw as I got to the end of the road with this one and continue to do so periodically.
To put this into perspective, before encountering her, I saw two clinical psychologists, another psychologist and several counsellors.
I hope you understand I’m not using my story to try to beat you up, rather to explain that waiting for the NHS isn’t a good option IMO. I really would look again and find someone, who has excellent boundaries to see if they can work with your elder dd. Perhaps psychodynamic therapy didn’t work with her because it was too short to really get to the root of the issue.
Your other dd probably has PTSD as well and I think all of you would benefit from therapy to be able to create good boundaries with her and move from this disordered family dynamic. My thoughts are that if you don’t, you may have to accept that your children will not be able to be in the same room as each other and may even not want to be in contact with you.
I am sorry what I am saying is so strong and coming at a time, when in reality you need calm and to collect your thoughts as your mum is so poorly. I actually think if they cannot find some kind of calm, the most obvious solution is to get your 21 year old to return and spend the remainder of the time with you in Spain. You could perhaps lean on mumsnetters to give you some support and help you navigate your relationship with her.