@Taryn45
If the family cancels the stay from their end, I am not sure they will get their Airbnb fees refunded, so that's certainly a disincentive for them.
Can you try reasoning with your existing guest? Is there somewhere you can store some of her things for those eight days so she doesn't have to move it all out and back in? Is there any nearby accommodation at a similar price where she could stay? Ok, so it's really inconvenient for her and those eight days may be more expensive for her, but you are giving her a great rate and she is the one whose plans have changed.
Have you tried explaining to her that if you cancel the other guests, you will not only lose your chance of being a Superhost for a year, but be financially penalised by Airbnb and automatically have a black mark on your reviews?