My in-laws are just like that. We have been married for 12 years, and not once did they come to visit us in the U.S., and they didn't even come here for our wedding. My kids are now 7 and 3, and they have never come to visit us to see them either.
My in-laws live abroad as well.
Needless to say, I think they are selfish. They do have the means to come and visit us, but won't.
At some point though... either they will need to come here to visit you, or you all need to take a trip there.
When my daughter was 6... my husband took her on the trip to see her family abroad. Just the 2 of them. She was old enough then to go on the long plane ride. For us, it takes about 23 hours, one way, to Europe. Meanwhile, me and my son, then 2, stayed home. We simply could not afford for all of us to go on the trip.
My husband and daughter stayed there 2 weeks. It was my daughter's 1st trip there. She got to see her cousins and Grandma and that side of the family. She, at that age, could understand and appreciate seeing her Daddy's family and enjoy the trip...and be able to handle the LONG plane trip. It was more appropriate, for her, at that age. She understood that only Daddy was going with her, that Mommy had to stay home with my son. It was okay, for her, at that age then.
I think, 18 months old, is too young. AND your Husband has NEVER been with your baby, for longer than 3 hours at home. How the heck does he think he can manage, on a long plane ride, by himself, and then handle the crying or feedings or anything, that your baby may need. He has not, at home, proven that he can be the primary caretaker for your baby, at home. Much less for 24 hours a day. Continuously.
Plus, you cannot take off of work. Anyone should be able to understand that. You can't help that. It is your employment. Jobs now are hard to come by with the economy. You can't risk losing your job.
Plus, can you all afford the trip? That is a big factor too.
The trip will not scar your daughter... but you will not be there. And she may or may not adjust well. She is young.
When she is older.... it will be better. When you ALL can go on the trip together. A Mom, deserves to be included in the trip. Not left out.
Your Husband... should be able to support your feelings on it. It is your AND his baby... not your MIL's. YOU are her Mother. If your in-laws really want to see your baby, they can come here. It is your Husband's responsibility, to talk with your in-laws, and see if they will come here, instead.
You are not being unreasonable. I know how you feel. I had the same situation. My MIL also said that MY kids should go there to visit her, and she thinks they can travel by themselves. NO. That is not an option. I am the Mom, and I have the last say. Period. And, it is just too expensive for us.
All the best,