How did he manage to put the Property into your DH's name - did your DH sign a Power of Attorney ? When a transfer is done the transferee must sign the document or someone else who has the power under a Power of Attorney. Did he forge your DH's signature ?
Have you done a search in the Land Registry to confirm that the property is indeed in your DH's name ?
Again with the Credit Cards - how was this done without your DH's signature ?
I would get a Tax Consultant asap. Get as much info and documentation together and go and see someone professionally.
They were all forged
Horrible situation for you both to be in. I cannot advise you strongly enough to go to a solicitor about this as soon as possible. Bring all of your documentation. Your FIL does not have to know about it. You need to look after your own credit rating and your own reputation. How do you know your FIL hasn't conducted other secret, fraudulent deals involving your financial details? Remaining passive and doing nothing might end up reflecting badly on you, indicating a degree of complicity. Get proper advice ASAP!
With regards to the property, only a transferor has to execute a deed. The transferee doesn't. So there could easily be property put into someone's name without their knowledge. It's common enough now with people trying to hide assets/claim lack of knowledge.
Would you just try and sell the property?
God, horrific situation for you to be in. Certainly don't know much about the legal side if things but I'd imagine if you wanted to extracate yourself from this it would involve charges against your fil. Did he do any of this to any of ye other brothers?
What I would do is to get an appointment with a solicitor, getting info from them. Then present it to the in laws and tell them they have two options: monthly standing order if a high value to get it paid off or you'll contact ye gaurds.
Who is paying the mortgages?
You poor thing, what an absolutely horrific situation. I'm going to be blunt though, your FIL needs to be held accountable. I would make an appointment with a solicitor ASAP and see what your options are. I would not be letting them jet off to Spain for a holiday, while ye are worried sick. You shouldn't have to pay back the money he ran up on the credit cards.
It's actually like a fiction novel