My friend knows my situation and understands the pressures of caring as we talk about it often. She also knows I am going on holiday on my own and that I enjoy travelling solo.
And yet..
My friend has now invited herself and her daughter along.. 
I just DO NOT understand how so many people on MN apparently get themselves railroaded into these farcical situations without speaking up before things get out of hand. Whether it's hijacking someone's week of volunteering at the Hay Literary Festival, or some cheeky mate getting you to pay for unnecessary purchases and coffees on shopping trips and not contributing to shared takeaway meals, always saying they'll pay you back but never doing it. FFS, what is the matter with you all?
Did the conversation not go like this? 'I've booked myself a little holiday. I am so looking forward to five days or proper respite with no caring duties and being completely alone to do as I please.'
Because that's how you've set the scene for us, so why isn't it how you set the scene for her - this friend who apparently knows exactly how much you need a break and how you enjoy travelling alone? What on earth did you say (or not say) that gave her the idea that her tagging along with her disabled DD would be even remotely welcomed, on your rare week of respite?
Why are so many people seemingly incapable of saying 'That's a nice idea but it doesn't really work for me because XYZ, so sorry, but no.' Is it really that hard?
I honestly don't know anyone in real life who would lack even the basic, entry level of assertiveness to deal with things like this without needing to resort to being rude or upsetting someone. And I don't know anyone cheeky enough to take advantage of that, or stupid or thick skinned enough to not get the hint when hinted at. Reading MN it sometimes feel like I inhabit a parallel universe to the one some of you live in.