Army Corporal vs. Marine Corporal: is there a difference in the mentality of each?
A lot of the duties Army SGTs had we gave to our LCPLs and CPLs.
Their is a big difference, CPL in the Marines is almost stronger than an SGT in the Army as far as responsibility and getting promoted to the rank, while CPL in the Army is many times given due to a SPC being senior in a section or squad and having the ability to be an NCO just not the points to be promoted.
A CPL in the Marines is promoted very stringently based on what is called a merit promotion something that doesn't happen every day as they expected to act in the capacity as any other NCO in the USMC, unlike a CPL in the Army who is not able to fit in with other E4s because they are considered an NCO, yet they don't truly fit in with the NCOs as they are just an E4.
I could go on and on, but I am sure your cousin has hit on these and many more I could cover as well, there is no real comparison on the subject to be honest with you.