My parents are rich too. Self-made. They still have to work, but have a very good lifestyle. They could sell their business tomorrow, and live an amazing lifestyle, but choose to continue working.
We're not rich, my parents money is not enough to give their children a luxury lifestyle. People sometimes assume that I do not have to work, its not the case. Parents money is enough to help their children with good deposits on a house, and provide us with jobs in the family business, if we so wish. They had too many children, for us to be in the position of being rich!
I digress. My parents insisted on paying for our wedding. They interfered on how it should be, as they were paying. We could have told them to get stuffed, but didn't. As the cost of say a 5k wedding, would be a lot to us, but inconsequental to them. We just accepted, that they paid, and they got a say in the day. The wedding cost them about 20k in the end. We got married, with all our family and friends with us, and no debts! Happy bride and groom.
I understand where your SIL is coming from, with interfering MIL. However once she decided to let the in-laws stump up, so has to go along with it. If we'd cared enough about having the day, our way, we'd have just paid for it ourselves.
The bride and groom are rude to put a £500 request. They could have just got married in Australia, paid for it themselves, or at least chosen a cheaper time of the year to get married.
I totally get where you are coming from with having to go to the wedding, we're having to go to a wedding in Italy this summer. Sometimes you feel obliged to go, and would have a terrible guilty feeling if you missed it.