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So the nephew is a US citizen but the niece is Greek. They attend college in England/Wales so will be coming from London. Oh, and now the nephew wants to bring his girlfriend - ok but we aren't paying for that plane ticket! At least the niece and nephew are certified to charter and she has raced small boats competitively since she was around 10.

So we'll be coordinating 5 people and dealing with transportation, provisioning, boat size and cruising ground restrictions due to visa requirements if we take the boat from the BVI to USVI.
Well the Greek niece needs to get herself a B1/B2 U.S. visa or you can forget about USVI and SVI. She needs to start now because if she doesn't get one, the plans change massively. If Nephew's girlfriend doesn't hold a US passport then the same applies to her.

Personally, I wouldn't be worrying about any of your other concerns. Yes, it can be difficult dealing with another mature couple and their needs as you've already found out. Organising a good time for 3 late teens, early 20's is really easy... just show them enough sun, sea, sand and alcohol and they'll have a great time.