Per her age, its fine.
She won't even realize looking back, that she missed a trip.
When my son was 2 years old, my Husband and daughter went on a trip to Europe to see his family.
It takes, from Hawaii, 24 hours to get there one-way and entails stop overs and connections. LONG flight. Even for me, I can't stand it. I get all claustrophobic and can't stand sitting for that long in a plane. And I am an adult.
So, because my son is SO active and would NOT be able to be on a plane for that long, only my Husband and Daughter went on the trip. Me and my son stayed home. It was fine.
They called us and we did Skype.
My son had no issues about it. Even at his age.
It was fine.
It would have been way to arduous and tiring for him, to travel all that way. He would have gone nuts-o on the plane for that long.
We simply explained to the In-Laws, that the flight is simply TOO long for our son. And WE know him best. It was not up to them anyway.
You are not terrible.
Just leave the baby home.
Its fine.
As you said, your In-Laws would be good caretakers or him. And you will line up some babysitters, if need be.
Just make sure they keep up, your baby's normal regular sleep routines and bed times and nap times. Otherwise when you get back home, you will have to get her back into her routine.
Write down, your child's routines for them and bed/nap times.
Go over it with them. And any other needs/rules for her and foods etc.
Its fine.
You can call your In-laws and do Skype too.
Make sure they have a computer camera to do Skype and set it up for them. Ahead of time. And teach them how.