Where do your in-laws live? or did I miss it in your post?
I'm a little concerned about the "eat only apples", as they do contain carbohydrate and may spike her blood glucose levels. Nor does it do anyone much good to live on just one food for any length of time. Apples are very acidic.
If you can, do try to find out exactly what they have told your mother-in-law. Particularly which Type has she been diagnosed with. Even Type 2s may need injections at first, if they've been undiagnosed for a long time and their blood glucose is very high.
With all types of diabetes, part of the way to manage the disease is by controlling carbohydrate intake. That means smaller portions of any grains, pasta, bread, all flour products and root vegetables; also very sweet fruits, and bananas, are best avoided, or eaten in very small quantities.
Have a good read around on here and see what information you can find; also Google "Dr Richard Bernstein", who has written a book for Type 1s but which is also very relevant for Type 2s. I think most of the information is also on his website.
My feeling would be that your mother-in-law shouldn't travel a 24 hour journey until she is completely happy with her blood glucose control and understands exactly what she is doing. She will be very tired after such a long journey and that won't be good for her blood glucose. She doesn't want to have a hypoglycaemic attack on the plane.
If she has already taken out travel insurance, there may be a problem if she doesn't tell the insurer about her diagnosis before travelling.
All in all, if your wife can travel, it might be best for her to go to them; save the in-laws' visit for another time.
Viv 8)