The older one may not necessarily be the “strongest” in mind. A big “EXCUSE ME” in a low tone if it gets too much. Not looking at either of them or saying anymore when it is happening. When older dog tells off does puppy stop ??? What age did you get puppy at ??? Maybe not with mum long enough to learn manners..
My older dog always put the pup in its place,he never hurt him but would put him in his place,leave your older dog to get on with it,it sounds like she is doing a great job 🙂