So what is the likely outcome..
You say: Mum and Dad, I love having you come to visit and join us on holiday but after 4 years doing this, I am feeling the burden of hosting and paying for everyone and all the work that it involves. I would like to change the dynamic a bit so that I feel we are all putting something into the holiday. Do you think you might consider renting the place next door with DB and their family so we can share the load a bit?
Mum says.. sorry darling, we don't want to pay for a whole week and what happens if it rains? We will come and visit you if it's sunny and we will see DB and family will come to visit us..
I notice that you are talking about your Mum and your sister in law, where are the men in your family??? I assume it's your brother... talk to him!!
If you said the same to your brother.. is he likely to think that as he has his big holiday booked for next year, he won't be paying for a cottage to see you and therefore, the 'tradition' of you hosting everyone relies solely on you doing all of the 'wifework' as well as paying for everything and it will all fall apart unless you continue.. and your children will loose out because they won't see their cousins and you won't get this wonderful family time you enjoy?
We have similar in our family and somehow everyone seems to feel like they are dancing to everyone else's tune.. kids are 18 down to 8. We did drop in the ... 'well you know where we live' line 2 years ago and were AMAZING hosts.. but sadly Covid got in the way of us seeing if that changed behaviours when we next visit.. It is a nightmare, despite loving watching all of the cousins hang out and play together and all of us having fun at the beach, the down sides for us are pretty full on.. who knows what reality will look like a year or so down the line when travel becomes easy again, we are in 3 different countries so it really does put a massive strain on relationships..
I have no easy answer but if you end up being on your own next year, spend the extra food money on the most wonderful meal / extravagant day out or a fancy new dress and straw hat to take wonderful photos in.