Ok, so I honestly don't know where else to turn.
My in-laws (as usual) are driving me mad!
The issue at hand is that my MIL, BIL, and my very own sister spoil my oldest daughter to no end, and it's causing DD to misbehave and ignore DF and I when they're around.
A few examples:
1- DD starts to cry when MIL is leaving...MIL gives DD chocolate to "make her feel better".
2- We do discipline DD when these people are around, but it's only to have them undermine us with "oh, leave her!" or "we're just playing"...BIL goes as far as mocking us to DD with comments like "Oh, mom and dad don't want you to have fun!"
3- They all act like they are ENTITLED to decisions regarding their upbringing regarding: naps, babysitting, daycare, nutrition, discipline, activities, education...can you believe that?!
4- They sneak snacks behind our backs because they know we'll say no. They try to get us to skip their naps, or give us attitude when we put them down for a nap, they indulge and give in to DD interrupting their conversations, rough housing, letting her hit (laughing it off or ignoring it completely) and allowing games we don't allow (screaming in the house, throwing the ball upstairs, jumping on the couch, ect...).
5- When we try to take some one on one time with the kids to dissuade the misbehaving/bad attitude, they come in the room or follow us, and ensure our time is short lived because "this is their visit".
The major problem: My sister and BIL (who are also engaged, lol I know, it sounds weird) are getting married in Mexico in 17 days and we're going to be stuck at a resort with these people and our children,...and as you can well imagine, we're dreading it. We want so badly to have a good time with our kids, but foresee sooooo many issues with these other people.
Luckily our youngest daughter isn't falling into the trap, because we've nipped that one in the bud very early on. Unfortunately, with our oldest, we caught it too late, and now have some MAJOR "reconstructive surgery" to perform! lol
HELP! We need advice! I feel so helpless, and completely inadequate as a mother for letting it get this bad! In our defense, we HAVE had the talks with these people, and all I can say...They're evil

. They don't care. They don't try. They argue back. They're righteous you-know-whats. They mock us when we confront them, or turn it into a huge fight.
I KNOW it's going to be an issue at the resort. I know they're going to follow us around, and ****** at us when we take the kids away from them to spend time with them on our own, and outright criticize the way we do things, and defy us...
Is the answer as simple as "who gives a crap, just do what you feel is necessary with your children"?
What's the best approach? I don't want to ruin their wedding by starting a huge fight...but in all honesty, at this point, I almost couldn't even care less if I do! *sigh* Any advice would be greatly appreciated!