This is a scam Kiora and we had the same thing with my mother. I am sorry to hear you are having to try and deal with it as it is very difficult.
Your MIL is now on a list and they do pass these to other companies so getting rid of them is difficult but we did manage it in the end but not before my mother's bank had taken her card off her and stopped her using checks. She kept telling them (so they said when we saw them with her) that she had no relations she could talk to and, when a lovely neighbour caught her in tears (because of the bank) she would not talk to me but only my brother who is too far away to do anything. He did get in touch with me but she still did not really want me to sort things out.
Even then it took us ages to convince her that this was a con. We sorted her money out fairly quickly and I wrote very strong letters to the companies but she still hears from them and their clones occasionally. There is very little you can do about this because they work from outside the UK.
We did use this to talk to Mum about power of attorney but I have a feeling your MIL would not accept that. It is something we should all do when we are very mentally fit because I have a feeling this stage is one many go through.
We decided there were many components to the whole thing. First, Dad had managed all the banking, etc. Mum was brilliant with the housekeeping but that was basically cash. She was quite confused about the bank account, loan account (the bank had set this up for her) and the card account. Her basic maths is very good (she was born in 1920 so schooling was not as it is now) but she has no real comprehension of cash flow.
Second, she was very lonely after the death of my Dad - much more than we had realised. We had to put more visits in place and did try and sort out groups she could join but, like many, she didn't want to do that.
Third, sadly we now realise she was in the early stages of Alzheimer's and had a strong need to feel in control.
I am not sure this will help but, hopefully you will get something sorted out. It is far from easy and I do think there should be more campaigning about these companies.