That's an interesting spin on it. It would certainly explain why only the player's mother is present in the game. One sibling stayed with each parent after a divorce.

Only it must have happened so young that neither remember the split. Makes you wonder how USUM Mizuki "chose" to stay with her father. Maybe they were put on the ground like two puppies and one crawled to each parent. The pair was then split; Mother took SM Mizuki to travel the world eventually winding up in Alola while Father and USUM Mizuki remained in Kanto. Neither had contact with the other parent afterward nor were they even told about each other.

That must have been a nasty divorce. Uncle Kakui must have had a hand in bringing the sisters back together.

Tsumanne said:
That must have been a nasty divorce. Uncle Kakui must have had a hand in bringing the sisters back together.

Kukui trying to Parent Trap them back together

... I do wonder how Elio could react to his USUM twin too? Neither he could fight his "dopperganger", or he could just be cool with it and say that he didn't even knew he had a bro...