Stropps, I feel for you. We should collectively write a book about teen dds. And invite them to write a parallel version, like those parallel texts for language learning. It is awful - for us and for them.
Lalsy, your post about your dd really struck a chord with me. Ds seemed to feel that, as the elder child, he could never admit to any vulnerability, incompetence, or not knowing things. He thus could not ask for help. It was like the responsibility of the family lay on his shoulders, and his own needs must never be confessed. I really hope we've sorted that one out now. He is very pleased that at his current uni, students are far more incompetent than he is.
MI, I know it's stating the obvious yet difficult to achieve, but if you/your dsis can get your dmum's local Age Concern etc involved it will help. With my dad, it was also a difficult thing of whether to ask the local hospice for help, because obviously for most people, hospice means terminal, but that isn't the case in reality.