"Twas ever thus."
LOL! So true, MawBroon!
Willa, I think both pps have good suggestions. You need to let dd know that you and dh can't put up with such an irregular, everchanging schedule. Use your age as an "excuse" if you want. Tell her she and sil will either have to make other arrangements in the future, as MawBroon suggests, or agree to some ground rules like the ones Anya has posted.
If they get mad, that's unfortunate, but oh well. Kids change as they get older, sure, but that just means that parents have to do some things differently, like maybe travel less and stay home more. But that's for dd and sil to figure out. You just need to let them know what their childminding choices are.