There were family repercussions for a few years when I missed a wedding because I was 8 months along, and the Dr had said to not travel more than an hour from home, and the wedding was a full day's drive away. I was 8 months along, I was feeling huge and ungainly (although not sick) and I didn't WANT to travel hours and hours. I think I called the bride and told her I was really sorry, but I can't make your wedding, and told her why. Her sister was offended, but the bride was not, because she understood.
All family drama aside, it's not fun to travel when you're 8 months along. It's hard to get IN a car, it's ungainly to walk, the body is not very comfortable, (I never had to navigate a bus while pregnant, and doubt I would have loved the experience) and you can't do much for very long. I think the OP deserves the benefit of the doubt here in not being willing to do all the travel to see her sisters. I think if I were in her shoes, I'd do much the same - I'd hunker down and not move. I'd say sorry, but I'm not doing that, and I'd mean it very sincerely. The list of things I was willing and able to do when near the end of a pregnancy wasn't super long, and travel was not on it, nor was going out of my way to accommodate non-pregnant people who didn't go out of their way for me at other times...