CONGRATULATIONS!
And good luck! I agree with Jackie... BE FIRM, BE BLOUNT, AND DON'T FEEL GUILTY!!!
This is YOUR wedding... NOT THEIRS!! It's great to get advice and help, but when it comes down to it, you do what the two of you want... if it hurts someone elses feelings, then too bad... what's their beef anyway? They get a free party and free food... and the only thing that matters is that the two of you (& the officient) show up!
Even if they pitch in on some things, it should be out of the love for you two... not so they can poke their nose in your business.
Just remember, you can't make everyone happy... but you can make the two of you happy!
Ok, off my soap box now...
Congrats again and good luck! I'm sure it will be wonderful!