Among SUVCW membership qualifaction # 1:

...(or directly descended from a brother, sister, half-brother, or half-sister of such Soldier, etc.) who was regularly mustered and served honorably in, was honorably discharged from, or died in the service of, the Army, Navy, Marine Corps or Revenue Cutter Service of the United States of America or in such state regiments called to active service and was subject to the orders of United States general officers, between April 12, 1861 and April 9, 1865;

http://www.suvcw.org/member.htm