How can my brother request to lift a criminal detainer from VA after he has already served a sentence in GA ? He also has a detainer with ICE.

I am not sure from the question whether the terminology you are using is correct so please keep that possibility in mind.  But as a general principle if the Commonwealth of Virginia has filed a detainer it is because the Commonwealth of Virginia has a case it wants to bring to Court against someone.  Thus, unless and until the Commonwealth either drops that case or the case is successfully challenged in Court or otherwise concluded, they will keep the detainer in place until the person they want is brought before the Court.  The ICE detainer is somewhat similar in that it basically means that ICE has a matter that they want the person to answer for and thus, until that matter is concluded the detainer remains outstanding.