So I [16M] asked one of my friends [16F] out and I'm not sure what to think about her response.

So to give you some background:

This girl and I have been in the same class for 2 years and been really good friends. I'd say we're pretty close, close enough for her to sleep on my shoulder at least. Well, over time I've developed feelings for her. She probably know this (her friend know and has most likely told her), btw.

So I decided to ask her out. I sent her a snap asking if she had a good time in Austria(she went there on a trip last week). After that conversation was done I asked her if she had seen Deadpool yet(she was super hyped for the movie). She had not and said she'll "prolly see it with her sister". I told her I had two free tickets to any movie I want(which I do, thank you SF Bio, you're awesome) and asked if she wanted to go with me to which she answered she had promised her sister to go with her.

Pretty normal, right? So what's the problem. The problem is that I like to overthink things. Most people would probably just accept it and think they should have asked her earlier or something. But I look at it and see that she changed from "I will probably see it with my sister" to "I have promised to go see it with my sister". Why? Did she just say that because she didn't want to go with me?

Also, girls, if this was you and you did ideed promise your sister, would you break that promise for a guy you like?

tl;dr: Asked a girl out to a movie and she said she's already made plans with her sister to go see the movie.