Clearly, they will break in if there is no one there when they go to repossess it if they have not been given the keys. But they will then raise a charge for the repair work to put right the damage they have caused breaking in, & that will be added to all the other charges that will be made. The total, together with the outstanding mortgage and interest, will the charged against the account & deducted from the sale price when the lender sells the house. If they then have any money left over it will be returned to your brother in law, but if there is a shortfall they will chase him for it as he has a legal liability to pay it. So it could be in his interest to reduce their costs by making sure they have the keys (& by being present to make sure they do use them & don't break in anyway!)