Can i claim my sister and brother if my mother already claim them but i support my mother too

to be able to claim your mother, her gross income would have to be $4,300 or less and you would have to provide over 1/2 of her support. if you can claim her, she can't claim any dependents (a dependent can't claim dependents). to be able to claim your brother or sister the same rules would apply to each. gross income under $4,300 and you would have to provide over 1/2 the support for each.   if you can claim any of these people you will need to paper file because your mother already claimed them.  once your mother's return is fully processed, she would have to file an amended return removing the dependent(s) you claim.    if she doesn't this could delay her's and your's refund, if any.    if she fails to amend at some point you and her will get a notice from the IRS asking each taxpayer to amend. if nothing is done the next letter to each from the IRS asks for proof that you are entitled to claim them.  the one providing the stronger prof gets to keep them the other gets a bill usually with penalties and interest.