My younger sister is 28 years old. She lives in my house and I pay for more than 50% of her expenses (food, clothing... everything). Her income in 2018 was $7,000 for the year. She is a waitress.
Can I indicate her as my dependent or not?
No, you cannot. She would fail the gross income test, which is limited at $4,150. If she earns that amount or more, she cannot be claimed as a dependent given your relationship.
The IRS provides a tool to help you decide if you can claim somebody as a dependent. You should use that tool to plug in specific information to verify that you cannot claim her as a dependent.