I accidentally let a friend get away with invited herself all into my life

I'm not sure how to handle this.

I have a friend...only child, a bit bossy, emotional/sensitive, likes things her way, and she fights with everyone and then wonders why.

I invited her to spend the holidays at my parent's house (thanksgiving and christmas both). She does have a lot of good qualities and has stayed at my parent's before and has been more fun, relaxed, appreciative, etc than she usually is. She has no family and few friends in NYC.

A few weeks ago, she asked me what exactly we were doing for Thanksgiving. I told her my parents would pick us up to go to a friend's house upstate, then we'd go back to my parent's that night. She did ask when I was going home, and I said "I don't know, probably Sunday" or something like that.

I spoke to her today, and she started telling me how she made plans to meet up with this guy who's from my hometown and he was going to take the train home with us Sunday. I was like "uhh...well I don't know if I'm going to stay until Sunday". She was all disappointed sounding and was like "Well, when's the earliest you'd leave?" and I said "not before Friday night".

Also, Zach has to go to Cape Cod, and I wanted to go, but he just told me he has to leave Friday early, like by noon.

I don't know how to tell my friend, especially since I feel like I let her believe she could stay longer. Do I have to stick out the weekend with her, or can I tell her I'm happy to be spending Wednesday and Thursday with her and she'll be able to celebrate with me, but then I've got to do other things??

Tell her definitely, cause IMO if you let it go this time, it will keep happening, and will be harder next time, and the cycle will continue.. on and on and on and on and on and on, and well you get the picture

I subtly sent her an email that was like "hey you should take the train at X o'clock to Y station, I'm probably going to Cape Cod Friday, if I don't I'll take the train home with you, CAN'T WAIT TO SEE YOU Kthnxbye"

Sounds good. Now STICK TO IT.

I just realized how grammatically wonderful the title of this post is

I'll be honest, and not knowing the background etc I may be way off in my assumption, but if it were me, I would assume we would spend the whole time together (well, not connected at the hip lol) and be travelling home together. Not that it should be a big deal if you arent going to...but I just think that would be the natural assumption.

I would kinda assume that, too. So, I'd say definitely tell her different and then stick with it so she knows