You will be able to do it. It will suck, but you will do it.
When our twins were 11 months old we flew:
California - Australia (3 flights) and then
Australia - Pennsylvania (4 flights, 30 hours)
Recently, at 19 mo we flew:
Pennsylvania - Hong Kong and back (2 flights each way, total about 20+ hours?)
The worst, by far, was the Aus - PA 30 hour deal. It was just plain old too much traveling with no break. The twins were lap babies, and I did not sleep at all. I was delirious and actually vomiting on the final flight (handing a screaming child to strangers so that I could run to the lavatory. so.not.fun). So, for you, I would ask how many flights do you have to take to reach your destination? For us, the fewer flights, the better (for ex, the NY-HK flight is 16 hours, but at least we didn't have to change planes, which made things easier). If you have a lot of flights and you don't function well without sleep, I'd consider spending the night somewhere en route.
Secondly, if there is any way you can swing it, I'd buy one seat (or two) for the twins. It is really very rare these days to have any extra open (free) seats on international flights. We bought seats for the twins on our recent trip, and it was worth the extra money for comfort and sanity. We took their car seats with us, and they actually sat in them quite a bit.
I second it: Magnadoodles are the greatest invention ever for toddler entertainment! You should be able to find mini ones that have the 'pen' attached to the pad with a piece of string.
-Changes of clothing (we take PJs to change them into to simulate bed time, not that it necessarily works, but...!)
-Maybe even a change of clothing for adults in case a wee one gets sick all over you
- Coloring books and crayons
- Books they enjoy reading
- Make use of the personal TVs on the plane and don't feel guilty one bit for it
- Take any comfort items (blankets, etc)
- Snacks, water, snacks, water, water, water, water!
- Make sure both you and your DH have access to things you might need such as diapering equip, snacks, & toys, whether you do that before take off or if you pack the same essentials in two different carry-ons. In short, be well-organized; it's one less thing to worry about.
- Go with the flow. The airlines staff will likely be cranky and not helpful, but the flight can only last for so long. Soon you'll be having fun with family in India!
- We carried the twins on our backs/chests through the airport & security, which was nice, b/c we didn't ever have to take them out of the carriers (even though security). We did not bother with a stroller.
- We took toddler backpack leashes but didn't use them. If your kids are the running type, the leashes might come in handy.