Let me start off by saying that I too believe that it is none of your brother's business. And yeah, that kind of thing would have been better suited for a private message. Having been the daughter in a situation eerily similar to this, I would like to say one thing...don't let this hurt the relationship you have with you daughter.
My dad met his current wife online. They talked for a couple months and then he went to visit her. One lived in CA and the other in OH. When he came home from that weeklong visit, they were engaged. He then turned around and invited her to a family holiday at my house. This would be the time that my siblings and I would meet her for the first time, less than 2 months before the wedding. We were all concerned for our dad because everything was moving so quickly, but instead of listening to us, he got very self defensive and brought her anyway. The manner in which this happened caused a lot of distance between my siblings and I and our dad.
Now, I know the situations aren't completely the same. You aren't engaged. But they are similar enough for me to say that I think you should talk to your daughter and really find out what is bothering her. Actually listen to her before getting defensive. Also, I applaud your girlfriend for taking a step back. It is important for you to visit with your daughter and grandchildren. And lastly, if you shorten your trip, be very careful on how you explain this to your daughter. My dad rarely comes to visit. When he does, it is usually becuase he has a business trip and needs a place to stay. The holiday that my father came to introduce us to his now wife, he left the day before the actual holiday because she wanted to get home. This felt like she was making him choose sides and he chose her.
I wasn't going to post, but I figured it may help to hear from someone who has been in your daughter's position. You may not care or agree with anything I had to say, and that's fine.
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