SIL wedding abroad. HELP!!!

I'm married to a kiwi (New Zealander) and he's currently living in NZ and I'm living in the US so we've doing a lot of traveling for the past year (we've been married for 3 years). I just got back from NZ a few days ago. While I was there, his sister gave us her wedding invitation which will be in November. I'd love to go but flying to NZ is expensive and like I mentioned, I just got back from NZ 4 days ago. I feel so bad for considering not going. The wedding will be a very small wedding. Small ceremony and dinner at a restaurant afterwards. I feel like if it was a big wedding I'd definitely go no matter how much tickets are, but being a small wedding and my SIL stating it will not be a big deal as they are just simple people and it's no big deal as her and her fiance have been living together for a few years now I'm not sure I want to spend around $1500 on a ticket only to be there for 4-5 days. I feel horrible for feeling this way but we're looking into having hubby move here and that's already going to be expensive. We are also going to start seeing lawyers to get the "green card" process started, so we need to save as much as we can. I love his family and would love to be there for the wedding, but I feel there are other priorities. I keep going back and forth and dont know what to do. We usually get tickets when they're on sale but they're still not cheap ($1000 when they're on sale) but with their summer starting in December, I really don't think I'll be able to get cheap tickets to go to the wedding. What do I do?!?!? I need opinions please!!! Thank you