I think they are definitely being unreasonable. It wasn't your decision for your sister to have a baby and to expect you to fly down to see them. If she had been considerate she would have thought about all this before getting "knocked-up". If they are so upset about it just tell them not to worry about it until your next in town at Christmas or Easter or when ever that is. That should **** them off real nice.
*thumbs up*
Funnily enough when I heard the news about her getting knocked up I said I'll just meet the devil spawn at Christmas... Until I was sent pics *sigh* I'm a big softie.