Hmmm...... Technically, I suppose Elsa would be dominant, per se, but that's not the right term, I suspect. Someone once said that in a relationship the bull is the head, but the femme is the neck. The femme can turn the head however it wants to go, control what it sees.
I don't see that kind of relationship here. Elsa is the alpha wolf, the control, if you will. She moves the pair on its long term course. Anna is the Omega, she institutes play and is the heart of it all. Without Anna there is fimbulwinter; endless, numbing cold, without hope, without light and without the desire for anything at all. Without Elsa, there is squirrelly chaos, but it would be frenetic chaos; doing crazier and crazier things so Anna wouldn't have to think or feel the emptiness.