The fact that there is current arrears indicates that the property has not been remortgages as yet, as the completion of a remortgage would have only occured if those arrears had been cleared.
If your charge was placed correctly, with HM land registry, then you would have been contacted.
You must check with the building society if the property is in danger of being repossessed and be actively involved in finding a solution with your ex - for your own sake. A repossession will stay on his Brothers credit file for six years and massacre his credit rating, as does arrears.
Sell the house quick, or get it remo'd and take your money, quick! Then tell his Brother to place a 'notice of correction' on your credit file explaining the arrears, as they will probably appear.
Good luck