Rachel should use simple sentences to explain the task clearly and effectively to her brother. However, she could also use compound sentences to break down the steps of the task into smaller parts for easier understanding.
For example, she could say:
"Hey brother, I need you to water the plants while I'm out. Just grab the watering can, fill it with water, and go around the house watering each plant. Then check to make sure the soil is moist enough and if not, water a little more. Remember to put the watering can back in its place when you're done."
This way, she is using both simple and compound sentences to provide clear instructions.