I'm still figuring out the Mamsource stuff, so I apologize for being 'late' with my response.
He is a bit young to understand the difference, but not too young, IMHO, to start teaching him the difference. My experience has been that motive is what drives my kids to do things sometimes. What is his motive? Is he wanting to help his younger brother and keep him from harm, or is he wanting him to get in trouble? The olders may feel the youngers are doing something they can't do or doing something they've gotten in trouble for doing before. At this age, I truly think your oldest is feeling protective of the younger sibling.
I remember my mother telling me that when my baby brother was born I would crawl in his crib & tell people 'NO my baby'. And I was always 'mothering' him. When my 4th child was about 18 months, one of her sisters was 4 and she would tell Rebekah when to take a nap, when to eat, how to play...and Rebekah did it! That didn't last too long, but much longer than I expected. I don't think Sarah was trying to get her in trouble but truly felt she was helping. I think your oldest truly feels he's being 'mama's helper' in all this.
Just keep working with him, letting him know what you think is safe for the younger one to do and that you love how he's a good big brother, but you're the mom and for him to let you tell his little brother no.
Hope that helped, you got some great advice from this great group of mom's