You decide it's too risky to change both of them. Your mom was packing last night for a trip last night - that's why she kept bothering you to do chores. She was on her way out the door when Amy arrived, so you figure you can be patient a little longer. Your dad and sister are both out of the house so you'll be free to transform Amy without anyone around.
Sure enough, after another 5 minutes of talking, your mom finally leaves. You hear Amy getting something to drink in the kitchen as your mom's car pulls out of the driveway. You hesitate just a moment before clicking the button.
You hear a muffled yell, then "What the hell ?!" from downstairs.