"Now, my Dad wanted to have a lunch or dinner with all of us before Christmas while my sisters are visiting from interstate. This we have been planning for months, and it wasn't until last month that I did something about it ... It is booked on the same day as the annual family Christmas get together, which my Dad, DH and I knew when I booked, ... I thought my sisters knew the date clash too and were ok with it.
Last week, both my sisters realised the family Christmas thing was the same day and suggested ... that we should go to that instead."
So you booked the lunch on the same day as the annual Christmas get together, fully knowing what you were doing and are then surprised it caused a conflict? Of course it caused one. The best way to avoid the drama would have been to have booked on a different day, or failing that to be flexible and suggest solutions. Some solutions would be, more to a breakfast, move to lunch the day before, move to breakfast the day before, or cancel lunch and go to the big Christmas family event.
It is the height of disingenuous to play victim of the drama that you caused. You can try to justify, we had discussed a date before and all that, but if you didn't want to force the issue, you would have told them that the dates conflicted and were you still on for that since you were going to book the reservation.