Threatening divorce is pretty excessive.  My sister hated those month long, distance visits with his relatives so once the kids where old enough to not need so much attention, 6 or 7, she just quit going.  She used the time to do things for herself, relax and work around the house and he got time to spend with the kids and his mother.  Her husband was a bit snitty at first but then seemed he also enjoyed the time apart.  If forced, month long vacations are divorce material for him, then  maybe time to get away from this unreasonable bully?