There are two sets of rules.
Under the rules for "qualifying child dependent", you can claim your sibling as a dependent if they are:
1. younger than you
2. live in your home more than half the year
3. don't provide more than half their own support
4. are under age 19, or a full time student under age 24, or permanently disabled and unable to care for themselves.
For this support test, it is not necessary that you provide more than half their support, only that they provide less than half their own support.
Under the rules for "qualifying relative dependent" you can claim your siblings if:
1. you provide more than half their total support
2. they have less than $4700 of taxable income.
Under this set of rules, it is not necessary that they live in your home, but you must be able to prove you provided more than half their support. You add up all their support costs, and figure out how much of that you provided. There is a worksheet to help you on page 16 of publication 501.
https://www.irs.gov/pub/irs-pdf/p501.pdf
For example, suppose the total you paid for rent, utilities, food and other expenses is $2000/month and there are 4 people living in the home. That counts as you paying $500/month toward each other person's support. You can also include amounts spent on clothing, entertainment, travel and so on. Food stamps and other aid counts as support provided by someone else, not you. If you add it up and you paid more than half their total support for the year, you pass this test.
