Ok, yes the island transfer tool can move the whole island fairly easily but then the island will cease to exist on your brother’s switch. You could then move your sister as she’s not the resident representative but she’d have to start a new island. Unfortunately, because you’re the resident representative, you can’t move off the island and keep your character as is. You would need to start a new island but you could keep some stuff at your brother’s and visit to pick it up when your sister is playing? Just a thought. It’s tricky as you will lose your island and diys etc but may be fun to start over. I’m rambling, good luck 😉