how old is too old?!

DigForVictory

Tempting though the "pony up & OK" ideas are, I'm afraid you're looking at a ruckus.
As you have the team you want, these are literally distant relatives & they are Not Part Of The Plan. Can your parents (especially the sibling) help with clarity as in "money's tight, it's a Small Do & we've enough women to buy dresses for already"?
Yes, it is Your Day. Your folks are supposed to help - and as siblings should know where there are weak spots, bodies buried etc. So apply a little pressure to your family with dear Aunt as a priceless wedding gift?
It's somewhat tough on the cousins, but think of them as a possble drop off point for regifting wedding rammel you didn't ask for/don't want/now want rid... (OK so maybe not a toaster for the littler lass.)