I had a friend from a muslim country and when his father died (his mother was muslim and from the same country) even though he was 12 years old, custody of him and his younger brother reverted to his fathers family even though they were in a foreign country!. They declined and gave legal custody to the mother as they did not want to separate the family.

But they could have claimed custody later at any time if they so felt. Maybe something similar happened here?

When it comes to a mixed religion marriage, also solve these kind of issues before hand and get it in the marriage contract!