I have read some research that has challenged the "pack" theory for dogs. However, I do think there is something to be said about the way Miles perceives you and your mother. I am assuming that you have been the one training Miles and he has probably bonded with you in that way. Maybe your mother could practice some skills with him regularly to improve their relationship?? It is funny that he knows that he can get away with more with his Grandma.
I hope you get some good advice!