I've never tried myself, but I suspect there's no special benefit to having two service accounts associated with the same Square account and that any additional accounts would each need their own separate purchases for games, expansions, and game time.

I believe the intended purpose is for one single player to have multiple platforms registered. I just play on Windows. But if I wanted to also play on PlayStation, I would need to use this additional service account option in order to play my same character via PlayStation as well.

Your sister should have her own separate Square account.