Next time, just relay your concerns to the person.

I imagine that they would be out a lot of money if they cancelled the trip and maybe that pushed her into going. MIL is a big girl, She made the decision to go. FIL was a witness to the state of her health and so was SIL. You should have no guilt and even if DH had said "maybe you shouldn't go" that would have stopped her from going.

I hope her surgery goes well.