Thank you for your question
My name is Clare
I shall do my best to help you but I need some further information first
On what basis have they done this and has your mother in law appointed anyone to hold Power of Attorney on her behalf?
Does his mother still have Mental Capacity at all?
Would she still have sufficient mental capacity to sign a Power of Attorney do you think?
You need to check with her doctor if he or she beleives that your mother in law has mental capacity.
If so then arrange for her to see a solicitor to sign one.
If not then your husband needs to apply to be appointed as her Deputy
He can then demand the return of the documents removed by his Cousin.
Please ask if you need further details
You can instruct a solicitro to send one - or I can help you write one for your husband to sign