You can be claimed as a dependent...by your sister (if she's older than you).  If your mom has already filed claiming you, your sister will have to mail in her return if she wishes to claim you.  The IRS will then send out letters to ask for proof as to who has the correct claim.  Since you are in school, your sister will have to prove that your move to her house is a permanent and not a temporary one.  She will also be asked to prove that you are sisters (usually each birth certificate).  Here is a useful page provided by TurboTax on this subject:  Click Here

Either way, if you are preparing a tax return for yourself, you will check yes on the question that you can or will be claimed by someone else.  You may then prepare your return.  Be sure to provide your Education Information to your sister (your 1098-T form) so that she can claim the American Opportunity Credit (if you qualify) on your behalf.  Here is an FAQ on that as well:  https://ttlc.intuit.com/replies/5358150