Pre-nup ? Maybe not as Robert said she owns 25% of the plane.
Seems strange for a plane to be owned by 4 people, 3 of which are blood relatives and one is not.
How on earth would they ever work out the logistics of using the plane in a divorce if she retained a 1/4 share?
She, especially, seems like the type of person to try to tie up it's use in every holiday or birthday, every child's vacation time, and such.
I have a feeling her use of the plane to make visits to her lover in the Bahamas was inconveniencing everyone and she probably wanted this asset as much as the homes or $$$. Perhaps she though if he mysteriously died, she'd get his half share and be able to commandeer it for a majority of the time. A huge status symbol for her in The Bahamas and a huge status symbol for the boyfriend, too.