That age-old issue - your non-Disney friends want to join you at Disney

[Kooks and Bay Bay]

I'd be annoyed that they want to "hijack" a day that you already have planned out.

Is there another day on your trip that would work better for you? If so then suggest it.
Otherwise just be honest and say that you already have this day planned for you and your sister specifically and make sure the friend knows going on a whim is not really how you do your vacations so if they want to join you it would have to be planned in advance.

 

[brockash]

I would love for you to join us but we already have reservations for things! Lets plan another trip together instead

This...also if they want to join I would just be very upfront with your itinerary and how you plan to tour. - If they think that sounds like something they want to do then they can, but I'd just be very clear with how you're going to tour and let them know it's really not for everyone.

 

[Miffy]

You should feel relieved, not even a teensy bit guilty.

A (former) friend of mine kept wanting to come to WDW with my sister and me and there was no way on earth it would've worked out, for about 3 million reasons. So I just told her no. And, btw, this isn't the reason we're no longer friends.

 

[MICKIMINI]

Don't do it! You'll stress until and during the vacation...stress when your friend is ruining your day and may not be able to get your mojo again during the trip. "The trip has been planned out and I can't change it now." Change the subject and don't talk about the trip.

 

[gmi3804]

"You probably don't want to do Disney the way I/we do Disney. You might want to re-think going, as much of our trip is already planned out with things we want to see and do."