I have flown with both my cats sedated and had zero issues, and that included my Mikey, who had had a previous stroke. However, I would never, ever fly them as cargo, only in the cabin with me! I've heard way too many horror stories for that! If MIL can't fly out to you with the kitty, I'd got he crate route. I wouldn't make the crate too big, though, just enough to fit a very small litter pan and food/water/towel. Many cats like a smaller space, where they can "hide" and feel safe rather than a huge one. Also, for some, a blanket over top to make it dark (the way you cover a birdcage) can help some kitties. Like people, they all have their own preference!