Deductions for medical

Yes & Yes. 

Those are  two separate questions.  But, in your case, both answers hinge on the support  test.  There is a special rule that allows you to claim the medical expenses of a person who is not your dependent; known as a "medical dependent rule"*.   

However it's unlikely he qualifies as your dependent if he has $18,000/year to spend on himself.  To be either a medical dependent or regular dependent, $18,000 needs to less than half his total support for the year.

A person can still be a Qualifying relative dependent, if not a Qualifying Child**, if he meets the 6 tests for claiming a dependent:

1. Closely Related OR live with the taxpayer ALL year  
2. His/her gross taxable income for the year must be less than $4200 
3. The taxpayer must have provided more than 1/2 his support
4. He must be a US citizen or resident of the US, Canada or Mexico
5. He must not file a joint return with his spouse or be claiming a dependent of his own
6. He must not be the qualifying child of another taxpayer

Nontaxable Social security doesn't count as income, for the income test, but social security money he spends on himself does count as support not provided by you, for the support test. Money he puts into savings & investment does not count as support he spent on himself. Note that a sibling is closely related so there is no requirement that he live with you at any time, during the year. But if you provided a home it helps your support case, unless they own the home you live in. If no one person (or married couple) provides 50% of the support (for example your siblings are also sending support), then a "multiple support agreement” (IRS Form 2120) can be used, to allow you to claim the dependent. https://www.irs.gov/pub/irs-pdf/f2120.pdf

The IRS has a worksheet that can be used to help with the support calculation. See: http://apps.irs.gov/app/vita/content/globalmedia/teacher/worksheet_for_determining_support_4012.pdf

See full rules at: https://turbotax.intuit.com/tax-tools/tax-tips/Family/Rules-for-Claiming-a-Dependent-on-Your-Tax-Return/INF12139.html

*If your close relative cannot be your dependent because he had more than $4,200 of gross income, but otherwise would have qualified as your dependent (basically that you & other family members provided more than half his support), then you can include in your deductible medical expenses any qualifying medical expenses you paid on his/her behalf just as if he/she was your dependent.

**If he is considered disabled, he may qualify as a dependent under the qualifying child (QC) rules instead.  But there is still a  support test to be met.  The support test, for a QC, is only that the child didn't provide more than half his own support. The support test for a Qualifying Relative is that the taxpayer provided more than half the relative's support.