Anonymous wrote:Practice saying "this is what works best for us! but you all go and have fun!" in your cheeriest voice. Repeat as many times as needed.
This, OP, this. Good advice. So is the idea of asking her not to bring it up and saying lets' agree to disagree on rules about sleep and food.
I'm surprised your sister has forgotten that a six-month-old will have hell to pay (and so will the parents) if the sleep and nap schedules get messed up just so everyone can do some outing. If she's really persistent, you might need to tell her with that cheery voice, "Hey, sis, remember when YourChild was six months old? You know that if I let MyBaby skip a nap or try to nap on the beach, I guarantee you'll be up when you hear baby fussing from being overtired at bedtime. That's not good for anyone's vacation, if we're all hearing a fussing baby who's cranky through the night. You go do X and we'll see you after!"
Maybe you can take your three-year-old to do some things with her and her kids while your spouse stays back at the house during the baby's nap? Tag-team it when you can. There is no reason that you and both kids and your spouse ALL have to do every actvitity with her and her kids.
In fact, I'd maybe offer to take her four-year-old and your three-year-old somewhere while your spouse is with the baby, and thus give sister some time where she can take her older kids to do something that they'd find fun but the three- and four-year-olds couldn't really do. Get her to mix things up more, if she seems stuck on the idea that "we all have to do everything together every time." Too much togetherness isn't great for anyone when you're together on vacation.