Sounds like your babies are very new, so a big congratulations. I'll start with your first item. Your family and friends can just bloody well wait. You've just given birth to twins, you're knackered, sore and don't need the hassle, so tell them to do one. You can just say "I am very tired, feeding isn't established and I'm just not up to it all yet, I will let you know when I am" and repeat. A christening doesn't need to happen just yet, if at all.
Sleeping 2.5 hours at a time is normal, and tag team waking is normal with multiples, but they will learn to sleep through each other's crying. A breastfeeding counsellor will be able to help you with your latch, making feeding less painful and more efficient.
Your older children will get used to the babies being here. It's hard on all the family when a new baby comes along, never mind two or more. You can spend time with them when the babies are asleep, even if it's just 5 minutes to read a book. Reassure them constantly and tell them that they needed just as much attention when they were babies and it's normal, but as the babies get bigger, they'll find them more interesting and you'll have a little more time to spend with them. As for the summer break, why can't you go? If there's no way, don't feel guilty. They'll still have a fantastic time with their dad. Yes, they'll miss you, but I doubt very much that they'll resent you for it.
Finally, make sure that you're looking after yourself as much as you can, eating enough, drinking enough and resting whenever you get the slightest opportunity. Oh, and it will get a bit easier, I promise! My triplets are nearly 15, my eldest son is now 21 and I'm still here and (relatively) sane.