1. Her locking the house may not be the ideal step which would have been suggested to her in the given circumstances.
2. Instead, application should have been filed before the magistrate for breaking open the lock of the front door of the house. (It should have been included in the said Order allowing her staying in the said house itself against prayer made by her well anticipating the trick played by her husband and FIL)
3. However, since the High Court has not set aside the order passed by the lower Court, she can ask the police to allow the door to be opened in your presence to ensure that you can enter in to the house and start residing there.
4. I have my doubt that they will ever like to open the door and the said order has been availed to ensure that their lock is not broken by you which will be considered as a coercive action by you. It is indeed a smart move by them to counter your putting lock on the door over and above their lock.
5. You can file a petition before the said High Court praying for a direction upon them to open their lock to enable you to enter in to the said house for staying and in case of failure, you can break open the lock in presence of police to enter the said house.