@LoveGiGi Grrr, this has really wound me up on your behalf (and I was already feeling pissed off), so I’ve 'channelled my negative energy' into drafting an email response to your CF SILs. It was cathartic - try it (but actually send yours!) I hope you DO decide to turn them away – you’ll certainly enjoy your holiday a lot more if you do (and so will your cats)! 
for having such shitty SILs.
Dear SIL1 and SIL2,
I should apologise for the short notice, but I’m afraid it’s not going to be possible for you all to stay in our home whilst we are away on holiday.
To be perfectly honest, the requests from you both for your families to come and use our home as a kind of hotel base for a holiday whilst we’re away came as a bit of a shock and I reacted in haste when I said it would be OK. It won’t be OK – I’m feeling really stressed just thinking about having 9 people staying in our home when we’re not there, not to mention all the preparation that would need to be completed to get the house ready for so many guests, and the extra work we would no doubt face on our return to restore our home to feel like ‘ours’ again. I already feel stressed at the prospect, and that’s not how I want to feel on our much-needed family holiday.
Another issue is space. We have 3 bedrooms which would be available whilst we are away (ours is currently being redecorated, so we’ve locked it up to avoid the DCs playing in there for now). PILs would have one, SIL1 and DH another and a third for SIL2 – there simply wouldn’t be space for the 4 children to sleep. It wouldn’t be fair on them to spend their ‘holiday’ sleeping on our sofas – particularly as we don’t even allow our own DCs to have sleepovers in the front room and they’d be pretty miffed if (when) they found out that their cousins had been allowed to – in their house!
As you all know, our cats are incredibly important to us, they really are members of our family and they are not used to having lots of strangers around. They are ‘House Cats’, which means they view our house as their territory and are likely to be stressed by having so many people they don’t know there. As they are house cats, they must not leave the safety of our house; they have not been vaccinated so any interaction with a different cat could be fatal to them. I’m sure you would all be very careful, but you (especially your DCs) are not used to stopping cats from escaping through windows, or when you quickly open the door a crack to let someone into the house – we feel that the risk of our cats being upset, stressed and the possibility of them escaping is too big to take. We have accepted PILs offer to cat sit, because we feel confident in their abilities to care for them as we do – plus with no DC about, there is a much lower risk of a door being opened without taking care to ensure that the cats are not in the vicinity. I suspect I sound like a bit of a sap, but we’ve even installed CCTV to keep an eye on them in the house whilst we’re not there, so I understand you possibly thinking I’m being a bit precious – because I probably am!
I’ve also being doing our accounts and we were relying on saving a fair chunk on our gas, electricity and water bills whilst we’re away. Obviously this wouldn’t happen with 9 people staying – in fact our bills for this period would actually be higher than usual! I’m not pleading poverty; I just want you both to see that the decision that we don’t want to have 9 people staying in our home whilst we aren’t there wasn’t taken lightly – there have been many factors we have taken into consideration.
One of the main reasons behind our decision is that we simply don’t feel comfortable having you all stay in our home whilst we’re not there. We usually only ever see you around twice a year, and that’s when we make the journey to your town. You last visited us 5 years ago and my Husband / your brother is hurt that you would choose to finally make the journey to our home in London, but specifically because we’re not there and he feels taken advantage of (as do I), as it’s clear that you were planning on using us (well, our home) as a free hotel. It was obvious that it is our home you were interested in, rather than our family, which feels brazen, cheeky and deceptive.
I am sorry to upset your plans at short notice, but simply would not be able to enjoy our family holiday without sending this email. I hope you are able to make alternative accommodation arrangements and I suspect we’ll see you at Christmas / when hell freezes over / never (hopefully). 
Off you fuck now, @LoveGiGi