Joint mortgages get people into almost insoluble difficulties.
We are trying to save our own credit ratings as we desperately need to trade up to a house of our own. We cannot swap properties.
Forget it, you won't be able to maintain your credit record. Check it out with the ICB - I would guess that it's already shot.
I don't really see any point in going to court to force your brother to sell the house. It will take a long time and cost a lot of money. He will probably start paying the mortgage towards the end of it, and the judge will leave him where he is.
Stop paying the mortgage
This will damage your credit rating but it can't be helped.
Explain the situation to Bank of Scotland and tell them that you will support their efforts to repossess the house.
Write to your brother
Set out your proposed course of action.
Set out the costs involved in a forced repossession and sale of the property.
Show him that he will do better by agreeing to the voluntary sale now.
As it is probably a cheap tracker, show him the benefit of making the payments himself to keep the house and tracker.
This may well end up in court, so you will need a trail of correspondence to show the judge.
What is your sister's stake in the house?
Did she do an agreement when she jointly bought the house?
Has she paid anything at all on the mortgage before paying the last year's money?
Does she "own" half the equity in the property or will she just get back the payments she made over the past year?