So sorry to hear about your wife's death. In the awful circumstances of your wife being killed I can see how researching the law on this was not your first priority.
But it might be helpful to note that even if this debt does exist you personally are not legally liable for repaying it and cannot be sued in the county court. Nobody is responsible for or obliged to pay their spouse's debts. (Unless you had formally guaranteed the debt, which is not the case here.)
As for the payments you have already paid, if your b-i-l claimed this showed you accepted liability for the debt you'd have a strong defence. You would say, correctly, that you made the payments in error because you thought you were legally liable, and stopped paying when you discovered that you weren't. And I believe you could then recover payments made in error, but that needs researching a bit more.
The legal liability to repay this (if it is a loan that is legally repayable - that's yet to be established) falls on your late wife's Estate. Did she leave a Will? If so it's the responsibility of the Executors to satisfy themselves that the debt is actually due, and if it is to pay it.
Who are the Executors? That may be you too, but suing you as Executor isn't so straightforward. Is there sufficient money in your late wife's Estate to pay this debt if it is actually repayable?
It's not at all clear that this is a loan that could be recovered in court anyway. The court would have to decide whether it was a loan or a gift between brother and sister. Your b-i-l would have to prove to the court that the alleged loan was made. The court will not automatically accept it exists merely because he says so.