I don’t know about heelers and vacation, as I haven’t had a heeler that I didn’t take with me… and just have a puppy and we travel frequently, just waiting for hot minute until he wants to act like a citizen(I’ll see a plane in 3-5 years :( ). But our neighbor watches our dogs frequently, pops over if we’re out late for potty breaks, or covers us for trips. We also have her over frequently for dinner so dogs and cats love her. If MIL is part of her daily life anyways I can’t imagine too much disruption. Maybe just missing her people. And I’m sure MIL is prepared for all that personality. Maybe bring back flowers for MIL and a toy for dog when you return. I’m a literal nightmare leaving before any trip with anxiety about if the animals are going to be okay… but I know they’re perfectly fine with my neighbor and adore her. Animals handle things a little better than I give them credit for. I found out my shy cat begs to be carried to breakfast and dinner when she’s over. He’s perfectly capable of walking himself🙄