Tell him to offer to completely pay for you too, then :P

He can't make you take her but maybe you can strike a deal with him if he's insisting so much? Sure you don't want to go without her, but if you must, maybe you can get something out of it. Since you know, you'd be giving up your free time.

This is horrible on so many levels! Is she a lot younger than you? Could you make a casual comment about how you're assuming your dad is going to donate the same amount in spending money to you? It might make him realise how unfair he's being. As for taking her with you, can you keep your itinerary to yourself? They can't book her on the same flight if they don't know when it is....

This is the way it works: if he pays, he gets to make the decisions. If you pay, you get to make the decisions. While dad might not be paying for your vacation, is he paying for other things? If not, he doesn't have anything to hold over you. Stand your ground. Perhaps he'll pay for you too. PS. I know it sucks to be treated differently than your sister but if he's not paying fir you in any way, he can't hold you under his thumb and tell you to do anything. Freedom may well be worth it.