Scorpius | Cassiopeia
He will, especially if you told him about the situation in your own time.
That sounds warm enough to me. Mine were always Grandfather and Grandmother, and they could be rather alarming. My grandfather used to complain to my parents if I forgot to stand up to properly greet him when he entered the room, and that sort of thing. But I am sure that your grandmother is just adjusting to the situation, and she doesn't truly mean anything by it.
Of course you must call me Aunt Cassiopeia if you would like to, dear.