First of all, your sister has to be a resident for tax purposes ( although she doesnt have a greencard, if she stayed in US for at least 183 days during 2015, she is considered as a resident by meeting the Substantial Presence Test )
Next, for her age, there are four tests must be met for her to be your qualifying relative for dependency
In general yes. Her legal situation does come into play unfortunately. Please see here how the IRS defines a qualifying relative.
Qualifying Relative
Many people provide support to their aging parents. But just because you mail your 78-year-old mother a check every once in a while doesn’t mean you can claim her as a dependent. Here is a checklist for determining whether your mom (or other relative) qualifies.