Okay, I have a horrible feeling that this thread may be getting a bit nasty.
Tink143 - I do not have children but I do understand your situation. When I first announced a destination wedding, my family were not pleased in the slightest. I only have my mother coming and we are paying for her dining for the week, her Disney tickets,
travel insurance, overnight stay at hotel and transfers. Now neither my DF or I have much money but neither does my mom, so we are happy to pay these things to help her come. She really appreciates it and it makes me feel great that she can now be a part of this. My brother and his new wife on the other hand have chosen not to come. The excuse was they couldn't afford it. Months later they turn around and tell us they are going to WDW in May for 2 weeks (3 months before our wedding) now that made me mad
I think your family should appreciate the fact that you are going above and beyond to make them a part of your day, by paying things for them, which you do not have to do, it should be appreciated.
However just because you pay for them does not mean they will want to go. There may be other reasons she has not shared as to why she isn't willing to go. I think you really need to sit down with your sister and explain how important this is to you, that she be at your wedding. You don't want this sort of agro hanging over you while planning your wedding.
Hugs to you

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Thats a little rude. They tell you they can't afford it and then go 2 months before your wedding!
I would be upset too. That's just silly.
I think that everybody has good points about this thread but everybody just has to realize that everybody is going to have different views on situations.
I had a similar situation. My aunt complained to me how they can't afford it.I said thats okay you don't have to go and I understand. Then she went into saying I'm selfish, nobody id going to go(everybody but her wants to), it's ignorant to make people pay to go to your wedding,she can't afford it and wants to be there. I felt bad but I can't change my mind for one person when the rest of my family is thrilled. She is a stay at home mom and her son is now going to be 3.She stopped working when he was born but because of that they had to sell their house and now rent a house. It took her a long time to get pregnant so I understand how she wants to spend as much time with her son as possible. Although her husband is not very happy with it and is always begging her to get a job because they need money.So she has a job interview this week to work weekends at her husbands full time job. I was upset with her for calling me selfish,I was not mad because they can't afford it. Also it's because she doesn't want me to do it Disney she wants me to do it on a cruise,lol.
I asked everybody,my parents and bridal party because I know that it's going to cost them money and I really am honored that they wanted me to do this and want to be a part of it.
I know that it's frustrating but understand that there is always going to be somebody who complains. I would be upset too by what she said to you so i think that you should sit and talk to her and explain to your family that they should appreciate and thank you for helping to pay for them. That is not mandatory to do that and is not in the etiquette books to do but it is a very nice thing to do.
Good Luck with everything