I would say that the timing is simply unfortunate, OP. Your DH and his family want to come together due to a dying relative. Even though there will obviously be a social aspect to it, they wouldn't be doing it if the circumstances were different.
I can understand why you are unhappy about him going, especially as it is such a long flight , but I really can't see any option and think you will just have to swallow it, unfortunately. If you stand in the way then he and his family could resent you for it.
When I was expecting DD3, now 14, my DH's Dad was in the very advanced stages of a terminal illness by the time I reached your stage of pregnancy. We couldn't predict what would happen, so just had to rally the troops and have as much support as possible in place just in case he couldn't be with us. We don't have family living locally either, though they are in the same country, so easier than it is for you.
As it happened, DD3 decided to arrive 5 weeks early, and whilst not ideal, it did allow DH to be here in support.
I did have to face the possibility though that I could have ended up on my own, especially as my sister was also pregnant at the time and we were even had the Sam due date!!
It is hard. Get as much support together as you can, and hopefully DH is back well before you go into labour.