V.S.
Hi S.,
I had the same issue as you have.... Only I was on both sides of the problem.
My sister (who I love dearly) was a horrible disciplinarian. My niece ran wild... No limits being set because my sister didn't want to "squash her spirit". She would climb on furniture, take food off anyone's plate, pinch and scream. She would run when called, making everything a chase game well into her preschool years. (Sher was actually expelled from two preschools!) I remember my niece at 2 1/2 years old, using my screen door to "grate" her handful of cheese. When I scolded her, and removed the cheese.. my sister freaked out at my "interference". After that we only visited at her house.
My husband has 4 sisters, each with their own parenting style. We are all very close and spend a lot of time together. Some of his sisters are more lenient than I, some more strict.. all of them seeing no boundaries when it came to dealing with the kids as a group. I do subscribe to the "it takes a village" approach to childrearing, but sometimes it got really intrusive. We'd find ourselves stepping on each others toes. aqnd needd a "family meeting"...
The solution was to teach our kids the concept of "house rules" In one house you could eat in the livingroom, in another house you could not.. In one house "inside voices" was a strict rule, in another it was a cacophony of playful kids.
I'd suggest setting up house rules for your vacation place before you go. acknowledge that your SIL (and any other parents attending) have different rules, and that you need to come to a consensus on what you can tolerate and what you cannot. Explain your expectations to your kids, and respect the agreed upon limits. Also I'd tell your SIL that if she has a problem, to bring it to you ( unless it's a safety matter) And tell her it's because she is alienating your kids, and they are becoming afraid of her. Tell her you don't want her to b th bad guy.. and to leave that job to you when she's around.