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.