I agree that these people have been takign advantage of you.
I would have expected that people knowing this was your holiday cottage would realise for themselves that sheets need to be cleaned etc but as they obviously don't , then I think you need to draw up some rules and let people have them when they ask to use your cottage, and have a set laminated and left in the cottage.
My BiL owns a holiday apartment in Turkey - he and my Sister are very generous about letting family use it - the arrangements are that you pay something for the use (this is variable - It helps cover electricity, water and maintenance costs, so it's usually a bit more if you go in summer when you are likely to be making heavy use of the AC, for instance)
They explain that the place needs to be cleaned (which includes washing the floors and emptying all bins) and the sheets and towels washed, before you leave. There is a list of how the various appliances need to be left (e.g. fridge unplugged and door open, washing machine unplugged, water turned off in lobby etc)
Food wise, the understanding is that if you use anything you must replace it, so that there is never less available when you leave than when you arrive. I don't think this has ever needed to be spelled out to anyone. I know that before I went the first time I asked whether there was anything which they didn't want us to use (even if we replaced it) but I would take for granted that you replace stuff you use.
This has worked well with my family, BiL's family, various friends and even friends of friends.
it's a shame that your family is trying to take advantage but I think you need to set some boundaries.
In light of the problems you have had, I think these need to include firm, clear boundaries:
"This is our holiday home, our use of it will always take precedence over any other requests for the favor of using the property. If we say no, we mean no, and if you try to argue then we will simply take you off the list of people we are willing to lend the property to / then your next request will automatically get a no as well.
Any request from anyone who has who has in the past failed to leave the property in the condition they find it in, both in terms of cleanliness and stocks food and other items will automatically be refused.
If any damage is done to the property while you are using it, you are responsible. If a friend or guest or yours damages the property, you are responsible. Any damage must be repaired by professional contractors proved by us and carried out to our satisfaction. If the damage was caused by someone who was your guest or friend then You are responsible fr the repairs. Whether you are able to recover the cost from your guest is a private matter between you and your guest"
I would also add something like
"renting a property in this area costs around $xx per week. While we don't want to have to start charging family and friends, we cannot afford to further subsidise your holiday by paying for cleaning, replacing times we have already paid for, or repairing damage caused. We would like to be able to carry on offering you the gift of free accommodation but can only do so if you treat us, and our property, with respect and consideration."