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You can claim your brother-in-law as a relative providing he meets the requirements.
To be claimed as a qualifying relative, the person must meet all of the following criteria:
Not a qualifying child - The dependent cannot be a qualifying child of another taxpayer.
Gross Income - The dependent earns less than the personal exemption amount during the year. This means the dependent earns less than $3,650.
Total Support - You provide more than half of the dependent's total support during the year.
Relationship - You are related to the dependent in certain ways. (he does)
Joint Return - If the dependent is married, the dependent cannot file a joint return with his or her spouse.
Citizenship - The dependent must be a citizen or resident alien of the United States, Canada, or Mexico.
As long as he meets all the above you can claim him.