You obviously care about your family so much.
I want you to think about something. A wedding is extremely special. It's probably one of the happiest days of someones life. For the married couple, and the family. How do you think they will feel if you aren't there. How do you think they will feel using the money gathered from your death? To be honest I wouldn't use the money. I'd rather have my dad, step dad or not, be there watching me marry someone I love.
Money is seriously not everything. I know it ties into lifestyles so dramatically and we build ourselves around and too these lifestyles, but I can guarentee you life is still a beautiful thing to enjoy and share regardless of the financial situation. Your wifes daughter isn't marrying for a grand wedding, she's marrying because she loves her partner. Maybe they won't have the "perfect" wedding, but they'll have achother, and they'll have you and your wife there too.
You don't know, they may find other ways of aquiring the money for their wedding aswell. THere are other avenues, other possibilities, and other ways to live and and enjoy life. Money isn't everything....