"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.