Your brother can be your tax dependent under two different scenarios.
He can be a "qualifying child" dependent if he meets all these tests,
1. Any age and permanently and totally disabled. For IRS purposes, this means unable to perform any gainful work, due to a condition that is permanent, or likely to last at least one year, or likely to lead to death.
2. Lived with you more than half the year.
3. Did not provide more than half his own support. Support provided in his name by social security counts as support he provides but other need-based aid from the state does not.
4. If he was married, he can't have filed a joint tax return with his spouse.
Or, he can be a qualifying relative dependent if he meets all these tests,
1. Can't be anyone's "qualifying child" dependent.
2. Can be any age or live anywhere.
3. Does not have more than $4000 of taxable income for 2015 (or $3950 for 2014).
4. You provided more than half his total support.
Note that the support tests are worded slightly differently. Under one test, he can't provide more than half his own support. The rest could come from you, other relatives, or the state. In the other test, you must provide more than half. Support provided by you includes a share of the rental value of your home, utilities, food and so on.
There is a support worksheet here to help you figure it out. https://www.irs.gov/pub/irs-pdf/p501.pdf
Now, if you can or could have claimed him as a dependent, there are some other issues to work out.
•You can amend 2015, 2014 and 2013 if he meets the tests. You can't amend farther back than the 2013 tax return.
•For any year that you claim him as a dependent, you can take an itemized deduction for medical expenses that you paid for, subject to the 10% rule.
•For any year that you can claim him as a dependent, you may be held responsible for providing medical insurance for him under the affordable care act. If he did not have qualifying insurance and was not covered by medicaid, you could be subject to a penalty.
•If you are single, then adding him as a qualifying child dependent will also allow you to change your status to Head of Household. If he is a qualifying relative dependent, he would also allow you to file as head of household but only if he lived in your home more than half the year.
