she lives with me and I provide more than half of her expences
There are two different situations to consider. I assume your sister is not disabled, and that you are unmarried.
First, suppose your sister is older than 18 and not in school. If you provide over half of your sister's support, and she is not filing a tax return with a spouse, and she had income less than $4,050 in 2015, you can claim her. If in addition she lived with you and you provided over half of the household expenses, you can file as Head of Household.
If your sister is under age 19 or under 24 and a full-time student, you cannot claim her because you are younger than her. A "qualifying child" must be younger than the taxpayer.
According to the TurboTax website linked below, you can claim your sister if she meets all of the following requirements:
* She lives with you
* She earned less than $4,050 in 2016
* You pay more than half of her support
* You are the only person claiming her as a dependent
It depends. Did you provide the majority of her support and she was not claimed by someone else?
If she lived with you the entire year, made less than $4000, and you paid over half her living expenses, and can prove all of that, sure.