Every person is different. Every relationship is different.
But, one thing that I think should be in EVERY relationship is caring.
My husband was at the airport HOURS earlier than when I arrived to make sure he was there on time. I spent hundreds of dollars on phonecalls because it was cheaper for me than him. It mattered of course because I'm not made of money, but I considered it worth it. We texted, or called, or chatted via webcam every day.. whatever was possible.
I personally feel based on your stores that she WAS/IS using you for visa fraud. I think she planned to get the GC and then leave you. I also think she was/is probably having relationships with other people. The fact that her sister is also in the country makes the threat of fraud worse... she has someone she's trying to get through. The fact that she called you simply to tell you to do the documents.. that sounds like fraud to me. She could have emailed you that or something... very weird.
I think you need to call and cancel the visa. Her sister could easily send her sister money to take the flight now. Find another guy to marry and then leave and have HIM petition her, or something.
Any case could be fraud or fake-love, in that we can never know for sure that someone isn't a super actor. We only trust that they aren't. However in this case it does not sound like fraud or even fake-love. It does sound like you are incompatible, or that your relationship never got the chance to develop fully. It sounds like you are not fully committed to this girl so a trip over to see her would probably not be much good. If you are are not sure though then doing whatever necessary to make a trip over there would be good. Do let USCIS know that you will not be wanting the visa used any more. If you ever are in a position to file for a K1 again in the future, I would advise visiting the girl a couple more times before making that decision. (USCIS might require more proof from you of a bonafide relationship the second time around.)
One thing I wanted to say, is that it is not clear to me that she doesn't love you. So be gentle to her in your treatment of her. Personally, I may have chatted online for hours each day and travelled out of my way to meet my love and spent too much money on phone calls. But that doesn't mean that it is the "right" thing to do. There is nothing wrong with not spending money on bad-quality phone calls, or not spending every evening in a crowded internet cafe, or even trying to avoid airport traffic. I know people who would always make it to the airport and also people who would not think they have to make it to the airport if it is in any way hard to do so. So I don't think you can assume she does not love you, and so I think you should be gentle with her if you break up with her. Not that there is a good way to break up with someone, but don't say "YOU don't love me."
I guess I'm saying, the incidents you tell us about show that the two of you are probably not right for one another, and would ultimately have problems were you to marry. But they do not prove that her heart won't be broken, so take care.
