I'm in the same position as your DD, but a bit older. I think my brother is probably worse in terms of behavioural extremes but your post just felt so familiar to read, so many parallels, especially the part about ending up like the girl in love actually, which people said to me too).
I'm now in my thirties and here's what happened with me. I tried for years and eventually cut my brother off completely because I couldn't cope any more with the constant calls, suicide threats, nastiness etc.
I love my mum but I've had to distance myself because she's so entangled with my brother. I moved away, met someone and started a family of my own.
I now can't ever go home because I absolutely won't allow my daughter around my brother because he's so horrible and my mum won't set boundaries on him coming to the house. For the same reason she won't travel much because of (insert his latest drama here) so basically she's a stranger to her granddaughter.
One day, OP, this will be you. A heartbroken daughter who can't have a real relationship with you because your son dominates everything. Maybe in the future some grandkids that you never see because you come as a package with your toxic son and no mother in their right mind would allow children into that environment.
And for what?
You're not helping him. Your son will get worse as long as you are enabling him. He'll keep at it and he'll drive your daughter away.
My mother is so miserable, it breaks my heart and I've watched my brother get worse and worse over the years because he's never known any responsibility or consequences. But equally, I'm not willing to set myself on fire to keep someone else warm. If your daughter is already distancing herself then it sounds like she's learned the same life lesson.
Seriously, OP, I am your ghost of Christmas future here. Don't make the mistakes that my mother made. You will lose your daughter.