If you can be claimed as a dependent, you must check the box on your tax return that you can be claimed even if you don't want to.
Your sister can claim you as a dependent if your taxable income is less than $4250 for 2020, and if she pays more than half your living expenses, and if you aren't married filing a joint return with your spouse. You do not have to live in the same home.
If you can't be claimed as a dependent, then you should be able to claim a stimulus payment on your 2020 tax return (if you did not receive one in 2019), make sure you do not check the box that says "I can be claimed as a dependent." If your sister does claim you as a dependent, the IRS will automatically investigate both of you, usually starting 6-9 months after you file.
