If $400 makes or breaks someone for a wedding in this case, they should hold off till they are more stable IMO.
That's not fair, really. Many people love each other and want to get married but can't afford much of a wedding. That doesn't mean they can't handle being married and should wait. If people waited until they were ready to have kids, the world be at a population of 0. My wife and I got married and were broke. We took our honeymoon on the cash gifts we received. Been together 14 years. Money doesn't mean you are ready or not.
As for the neice, I would do what I did with my little sister. I told them that I would pay $500 towards their wedding as our gift. I told her that it was not for bills or food or anything else, just the wedding. When she was ready, I would call the business and put down my card number. Otherwise, she has to wait. She has a history of being financially irresponsible, but there is no one else to help them out.