See IRS publication 501 page 19. An adult must live with you all 365 days to be a dependent (plus meet other tests) unless they are a "relative who does not have to live with you." That list includes "A son or daughter of your brother or sister" and son or daughter includes foster children, if they were placed in the home by a legal authority.
<a rel="nofollow" target="_blank" href="
https://www.irs.gov/pub/irs-pdf/p501.pdf">https://www.irs.gov/pub/irs-pdf/p501.pdf</a>
Update: I think I agree with
@bwa and this answer is not correct as it might apply to a former foster child. I think it would still apply to a current foster child for however long that relationship exists under state law.
