S.H.
What do your twins sleep in at home? If possible, replicate that on your trip, at your family's home.
If they've never slept in cribs now, they won't there.
Why not try the tents again?
OR, simply put a mattress on the floor of your room where you will be sleeping (or in the room the twins will be sleeping) and have them sleep on that. Meanwhile, making the room "kid safe" so they don't get hurt or break anything.
I would get 4 seats...unless, you can imagine having 1 child on your lap for the whole trip. How many hours is your flight?
Keeping in mind, that a child can't just be cruising around on the plane... AND if the seat-belt signs are on for most of the trip...well, your kids HAVE TO be seated.
When on your trip... I STRONGLY advise that you try to keep to your twins "schedules" just as back home...ie: nap times, bed times, etc. OTHERWISE... when you come back home, you may have a problem and your kids won't sleep the same way, the same times, again. This happens many times with babies or young children. SO... try to keep things routine in that sense. Or, you may/will have to get them back on "schedule" or back onto their usual routines... once you get back home. And it isn't always easy to do that. Keep that in mind.
At this age, just have them sip on something, or chew on something at take off. They are older now... so it should not be AS bad for them. If not at all. Or, ask your Pediatrician.
HOPEFULLY, if the flight is at their usual nap times or bed times... then perhaps they will sleep on the flight. If not, well it will take concerted efforts to entertain them... keep them busy. Talk with them prior, and tell them you are going on an airplane, how to act etc. They are young and may not understand like a 5 year old would...but you never know... some kids once on a plane, will behave well. Unlike at home. My friend's kids are that way.
Bring their favorite toys, their lovey's, their comfort things, books, all the usual. Play-doh is a nice quiet activity, paper and crayons.
Also make sure your kids are fed beforehand... some kids get real fussy when hungry. My daughter is that way. And bring snacks on the plane with you.
Make sure you visit the TSA website, so you know what you can/cannot pack on the carry on bags. www.tsa.gov
good luck,
Susan