Personally, I don't think you should HAVE to pay for anybody's way to your DW. We didn't for ours, but I did do a lot of the planning and booking tickets for the planes, van rentals, park/hotel packages and cruise (charged on their credit cards, and split expenses for some) so I didn't feel bad. There were around 25 people. It could have been more wedding planning time for me, but I helped out with that instead. Like someone else mentioned, they get a vacation out of it too, so don't feel obligated.