D.K.
Can I say first off, count your blessings that you have family that is involved and gives you downtime!! I have no family, my parents have passed, my ex inlaws live out of state and barely are in contact with my children.
Something I read in Dear Abby not so long ago, pick your battles with family. Is it really the end of the world your daughter gets off her schedule for a few days? Is it a big deal grandma gives her candy? I mean think of the bigger picture. You could of course present them with written instructions, explain very diplomatically that she can be really grumpy if she does not stay on her schedule and so on. If it won't offend them I recommend that, however the fact that the grandparents want the kids there and are more hands on, I would be so happy! My kids have missed having grandparents to spoil them (that is what grandparents do). As long as they are not neglecting her or she isn't in harms way, I say pick your battles. I have friends too that would give anything to get a break from inlaws and complain their inlaws are so not a part of their kids lives.
Hang in there, biting your tongue for your kids sake is worth it.