taking children abroad

[ebbony]

my brothers wife is threatening to take his children abroad if her new husbands gets a job there (France) he has joint custody. Children are 11 and 6. The oldest doesn't want to go and is very upset. the youngest doesn't understand. My brother is very active in their life seeing them everyweek for a day or two sometimes more and definate every other weekend. His ex always makes things hard for him and doesn't want him to have contact at all. What does the law state as far as her being able to take the kids and live in another country. Can she do this or can he do something legally to stop her.
Many thanks
JH