In most cases the child is considered to live with you during temporary absences such as college. However, if the child has definitively moved out and changed their permanent address (will be moving on after college, won't come home, has moved out for good, etc.) then they aren't considered to live with you any more. In that case, the aunt may be able to claim them, because a parent's sibling has the same basic legal rights to claim a child as a dependent as the parent themselves, assuming they meet the residency and other tests.
If the child lives in the aunt's home more than half the nights of the year, then the aunt can claim the child on the residency rule if they ignore or don't know about the "temporary absence" rule that lets you claim your child. You should clarify this with your sister before tax time, because this is something you want to work out before rather than after.
