She can't do it without your brother's permission
Removal of a Child from the United Kingdom
No one is allowed to remove a child from the United Kingdom without the consent of the Court or the written consent of everybody with parental responsibility, so long as a residence order is in force. However, a person with a residence order (but no one else) may do so for a period of less than one month without the consent of the other parent or the consent of the Court.
If you wish to prevent this step being taken, then you would apply for a Prohibited Steps order.
If you are the parent seeking to take such a step and you are being prevented by the other parent, then you would seek a Specific Issue Order, as outlined above.
Where there is no residence order in force, a parent should seek leave to remove the child by way of a specific issue order.
The person who is looking after a child, can only take the child to live abroad with the written consent of every person who has Parental Responsibility or with specific permission from the Court. The proposal for removal must be �realistic and sensible�, and must be fully supported by evidence.
http://www.mensadviceline.org.uk/Articles/chil dren-and-contact.htm