Fuck That Shit again.
I am a roll-your-sleeves-up-and-and-do-it mother of twins, and another baby now. I happily loaded by 2 month old twins in the car and drove 4 hours to stay with family for a few days without my DH. I have taken them on holidays, camping, to weddings and to funerals.
There is no fucking way I would take newborn twins to a wedding a few weeks after they're born
If it was your sister's wedding, and she and your family were willing to bend over backwards to accommodate and help, I'd still advise saying you won't know until the day. Given the helpfulness shown by your DP and the couple so far, no fucking way.
At an absolute minimum, you would need to:
a) go in your own car, so that car seats and double buggy are sorted. You can't fit two infant car seats in a cab while holding two babies, and you can't rely on the driver to do it correctly. You won't know in advance whether your seats will fit in the cab. You won't know whether your double buggy will fit in the cab. You can't retrieve two babies, two seats and a double buggy from the cab at the other end. I was driving 10 days after each of my CS, but not far. If you're not driving, someone else needs to drive you there in your car.
b) take a lot of stuff with you. For an 8 ish hour period, my newborn twins would have needed 15 ish nappies, a packet of wipes, two changes of clothes each, two muslins each, changing mat, a blanket each, spare hat/socks each ..... You will need breast pads, maternity pads, a change of clothes for when you are poo/vomit splattered or your milk leaks everywhere. I am not being melodramatic when I say you'll need a suitcase to store it in and a large bag to carry the necessary stuff around.
c) to take a double buggy with two lie-flat seats or two carry cots. Babies are not meant to sleep in car seats for more than an hour at worst. They need to be flat.
d) space in the ceremony room and the reception room, and any other rooms they're using, for you to sit next to your double buggy. With my single baby, I can park the buggy in a corner and carry him/bag of stuff to a seat, but you can't with twins. Wherever you go, the buggy goes too.
e) somewhere private to feed. I fed my twins for almost three years, and am feeding this one too. I feed on trains, planes, mountains and buses; in libraries, swimming pools and school classrooms. But a few weeks in to feeding your first, and that first is two babies? You need somewhere where you can get your boobs out with no thought to others. Where you can feed on or both depending on how you/the babies are feeling. Where you can feed a baby, and then wind it while simultaneously mopping the baby vomit back out of your bra.
As I said, I really am a nothing-will-stop-me mum, and I have always been very stubborn about doing what other mums can do despite having twins. But this plan really won't work. Having newborn twins is amazing (despite what I said up there), and you will just instinctively do all of that stuff and not care that you stink of vomited breastmilk. But you would be a special kind of lady if you could do it all in a pretty dress, away from your house, with little/no help, when your babies are small. I have yet to meet anyone that special, though I know lots of wonderful twin mum's.
Don't go. Simple as. Your plan with your mum sounds lovely (and manageable).