I have a similar situation - my sister's husband left her and the two kids (now 3.5 and 1.5) last December. It is VERY important to me that they be there with me. Matt and I are paying for them to be there with us. My mom and dad originally were not planning on coming b/c they "can't afford" it. I tried to see how I would feel about having a traditional wedding, and I just would not be happy about it. I also tried to think about how I would feel without my parents there, and that would totally suck, but I'd be on the beach and that is where I wanted to get married. I decided that my parents would do what is best for them, but I had to so what's best for ME. I didn't want to look back on my wedding day as another day that was about them. (The story for why they can't afford it is a little deeper and has to do with my problem brother, but I am not going to go into that here.) So I decided, after all the years of them doing and paying for everything for my brother, if they wanted to be there for me, then great, if not, then that's great too. I don't know if this helped at all, but I agree with Jamy - you need to do what YOU want, b/c you don't want to look back at your wedding and have regrets.
Oh, and they are coming now, so it's all working out.