Water changes are very easy to perform. She doesn't have to vacuum the gravel/sand in the tank to give your fishy friends new, clean water. Even just a 10% water change after you've been gone a week will ensure you will come home to happy, healthy fish. I would also make sure the mother in law isn't over feeding the fish. My fish act hungry 24/7 and I would worry about someone else watching them because they may overfeed them thinking they are doing the right thing.
If you are too concerned by someone else doing a water change, feed very sparingly, depending on when you are leaving and your current setup consider adding some live plants, instruct mother in law to add some Prime to the tank after you've been gone a week, and have fun on your vacation!