Back story here -in summary, my half-sister, aged late 50s, turned up to see my parents (same dad, different mum) without giving them any notice. She proceeded to be fussy about what she ate and drank, before announcing at 9pm that she and her husband had not booked a b&b so would need a bed for the night. My dad (who is in his late 80s) had recently been in bed after firstly having a fall and then an operation. Mum was exhausted from the emotional impact and from looking after him.
This unfortunately turned into a bit of a row between mum and DSis, which DSis tried to drag me in to.
Mum realised that the row was going nowhere so disengaged, which has prompted DSis to write to our dad saying that she will come to visit on his next birthday if he is "still around".
I know there is nothing that I can really do to make things better so will keep quiet, but I am just so cross that she would say something so unkind to dad. 