If you meet the tests to be claimed by your sister and she is required to file a tax return, then you can't claim yourself. If she can claim you, then you must indicate on your tax return (if you file a return) that you can be claimed by someone else. Your sister would not be required to file a tax return if she had no taxable income. Even though she didn't work she might have unemployment compensation or maybe investment income. There is no benefit for you to claim yourself since you will not owe any taxes and are not required to file a tax return.
The only reason you would want to file a return with only $900 of W-2 income would be to get a refund of any federal income tax that was withheld.
