I'm sure they will learn in time
To me, it sounds like MJ is simply trying to teach the little puppy the 'power of the pack' and show her the ropes. Puppies NEED to be taught how to play correctly, and often times the adult dogs will nip/bite at them, or bark at them, etc to show them how it's supposed to be done. Sophie is still very young... give it a few more weeks! Also, give most of your attention to your attention, rather than Sophie, to show MJ that she is not being replaced. Give her extra love, extra treats, etc.