For us, it depends on the issue. As far as eating goes, my MIL expects everyone to sit at the table. We do as well, and so do BIL/SIL, so there's no problem there. We do not, however, expect the kids to eat some of everything. My nephews know that rule is different for our kids than for them, and they're fine with it. (They're 4, 7, and 12, so a good range.) On the other hand, they've never had to take naps, but my son is beastly without one. So for that we tell him that "Aunt H and Uncle J don't say that G & L have to nap, but you do." He did squirm and fuss a little the first couple of times, but now he understands.
For the most part, though, I try to relax on the rules because I like everyone to be comfortable. My ILs also take the lead on some things. I won't let my kids push chairs across my hardwood floors, for example, but when DD was doing it last week, MIL said, "oh, it's fine." So I let it go. I think kids are pretty good with understanding those kids of differences if you're clear and matter-of-fact about them.
It's us: DH , DS ; DD ; and me . Also there's the . And the 3 . I . Oh, and .