I know he isnt supporting the family. Thank you! That was my thoughts but I wanted to make sure I covered everything. 

"Support" was an issue in the good old days.  The issue now is whether any of the siblings are providing more than half their support. No one said Mom doesn't own the house and/or collect child support.  Or have $100K savings in the bank.  So let's not get hurt jumping to conclusions.

Which test disqualifies Big Brother from claiming them as a qualifying child?  I think you at least have to reach the point where tiebreaker rules are considered.  Wasn't there something when they changed the rules, that said a nonfiler (Mom) isn't considered for applying tiebreaker rules?  Or at least, the rules only apply if the same kid appears on two returns. 

And is the 18yo a student?  Not that it matters.  

All I know is the IRS EITC Assistant tells me the 18yo gets about $4,800.

https://www.irs.gov/credits-deductions/individuals/earned-income-tax-credit/use-the-eitc-assistant

But I'm not as good a wannabe auditor as some people. 

You need to determine if the 18 year old is a dependent or not.  For example, did the 18 year old provide over half of his own support?

https://apps.irs.gov/app/vita/content/globalmedia/teacher/worksheet_for_determining_support_4012.pdf

If the 18 year old is not a dependent, nothing in your post indicates he can't claim his siblings.  If he can claim them, he would receive the benefits, including the refundable Child Tax Credit and the Earned Income Credit.

Good points, maybe it does smell better, especially with the new and improved definition of a child, mid 90's change I think. It still seems off but the EIC assistant liked the odor.