I think that the emphasis on 'child' is extremely important.  We also were scrupulously fair with our five, they all lived with us, and we're all made absolutely equal.
However, they are no longer children.
  Most of ours have become well balanced, successful adults. We have problems with my eldest son, and a complete breakdown, similar to you JRI, with our YSD because of her appalling behaviour. Both of these adult stepdaughters (yours and mine 🙄) have, in my view, completely resisted taking responsibility for themselves or their behaviour and have basically refused to bloody well grow up.  DH and I gave ours all the same 'tools' (as pointed out to us by OSD) but they have steadfastly refused to pick them up and use them,  preferring to try and force/guilt-trip us into whatever it is at the time that they want, and then when we refuse to do so, behave horrifically.
I was 60 this year, and the 'kids ' are aged between 30 and 36 years old. I refuse to go into my 70th decade dealing with this narcissistic nonsense.
When I  hit 30, I was expecting my youngest child, and treated my parents and father in-law with love and respect. I also recently cared for my late father.
The point I'm trying to make JRI, is that your SD is not behaving as a daughter to her father, or as an adult towards either of you. We grew weary and bored of YSD's tears, threats and tantrums,  and so should you.  You should expect,  and deserve a whole lot better. Don't accept anything less sweetie. ❤️

While calling the police is a requirement if her behavior escalates and she ends up banging on your door it doesn’t ameliorate the need for a plan that goes beyond getting her off your doorstep.

I can’t imagine any agency would see a nearly 90 year old guy with health problems and the beginnings of dementia as solution to her housing problems. Not even with a capable 80 year old woman as his backup. In fact, doing so would likely be considered elder abuse.

This needs to get formally tossed back to her siblings and her kids. They’ve got to be made to understand that you are 100% done. C clearly hasn’t gotten that message when her solution is to have the neighbors drop her off at your house.

As long as they see you as the default solution this is going to keep happening. They need to be talking to each other, not to you.