To do this, you'll need to trigger the family to take a photo by putting a fuse in the photographer's fuse box. Also, fix Emma's chemistry set in the greenhouse with a wrench from the fountain. After they finish with the photo, they will head inside for a family meeting. After this, follow Emma to the greenhouse to make sure she starts creating the poison. While she does that, head inside and distract Alexa and her body guard to give Emma enough time to poison Alexa's drink.