My dad's getting older. If he had the money to pay for me to take a trip with him without my wife, I might consider it, but I'd want to make sure she was okay with it, first, especially since we have children to care for. How hold is your FIL? Your husband could be looking at this as an opportunity to spend time with him while he is still alive. Or maybe he just likes the free trips. Btw, doesn't he have a job? His father is early 50's, always traveling for pleasure, never at home with his current wife. Husband does have a full time job, so his vacation days are being used for this instead of time with me visiting my family overseas also.
Taking a DIL along a trip would probably just not be the same in regards to what they talk about and how they interact, and maybe even the things they do. Do they camp or trek through the woods? They don't camp or trek and I would be into all of that, I want to be a part of my husbands hobbies and show an interest in them.
Have you told him how you feel about this?