This is indeed a very tricky situation and I sympathise with you. Having had similar situations myself although not with relatives but with people who purported to be 'friends'.
As has already been suggested, the best solution would be to come to an agreement with your brother in law to make repayments under a written agreement over a specified period of time. Both parties should sign the agreement. If you are happy to do this simply between you and your brother in law then thats fine. Alternatively get a solicitor to draw up the agreement. Personally I would go for the latter.
If this does not work then there is the ultimate process of going to www.moneyclaim.gov.uk
. This is a very effective way of making a claim for money thorugh the courts. It all web based and can result in the person owing the money getting a County Court Judgement against them in the event that they do not pay up all that is owed which will include the fees charged by MCOL (these are compaitively minimal if compared with the alternative of employiing the services of a solicitor to do this for you). Going down this route will undoubtedly leave a 'nasty taste in the mouth' but you will get your money back one way or another as the last thing anyone wants is a county court judgement aginst them. You can even go as far as sending in the baliffs to seize goods to the value of the debt, including the fees. As I said, this has to be the ultimate procedure to go through in order to get what is owed to you and will take a lot of soul searching on the part of both you and your Husband. However, it is strange how debtors always seem to take the view that they are being hard done by when asked to repay what they owe.
I hope you can resolve this situation in the most amicable way possible.