Pay an assassin to kill your mother in law. She will be too traumatised to want to go away.
He wants to go on the trip with his wife and enjoy it, without the mother-in-law. Not ruin the trip for everyone!
OP: don't do this, bad idea.
Pay an assassin to kill your mother in law. She will be too traumatised to want to go away.
Have you thought about going along anyway and just brutally murdering your mother-in-law while she's asleep?