We do this lots. Grandparents, sibbling plus family me plus my family.
Works well but there are some tensions that need to be managed. 5 children, one with significant needs, 1 younger than the core group. Different financial attitudes. Different alcohol attitudes.
I find that it works best if we all work out what the key things we each want are from the break and pre-agree the financials, not to the penny but roughly whats joint spending and what isnt.
We went to a ski resort. BIL wanted to go skiing, I wanted to go husky sledging (some had done it before), Granny wanted to keep warm, kids wanted to sledge, Grandpa likes to play building the fire. These core things could all easily be built into the holiday by working out which days would fit for ski sessions, dog sessions, booking accomodation that had views over the ski slope so Granny could watch from the warmth (actually with warm clothes she enjoyed the fresh air but it was her worry). We booked large enough accommodation we could have adult space, children could have adult free space and child with needs could have his own space.
Sibling and I divided up 2/3 catering so no one spent the week in the kitchen other meals we left free for people to eat out or cater at the accomodation.
We do summer breaks too, but this is in a catered hotel so as part of the deal we have an agreed evening meal time we all eat together. We breakfast together and have an hour or three together in the morning going on a stroll or to the beach. Afternoons do our own things - quite often end up together.
It's great for getting a bit of adult only time without it being like children are stuck with sitters, they barely notice because they're in their own space but with cousins and grandparents.