I know her mother had it badly in her hands and that it caused a lot of pain but MIL has always been fit as a flea she knits and crochets which I doubt she'd be able to do with arthritis
Perhaps she is able to knit and crochet because the medication keeps her arthritis from causing her pain and problems.
I only discovered them when I looked into her medicine cabinet and there are boxes and boxes of them
Most GPs these days will not prescribe paracetamol and advise patients to buy them over-the-counter as they are so cheap.
When I went to the GP with foot and knee pain I was prescribed Naproxen. However, that and other NSAIDs eg diclofenac, ibuprofen, are not routinely prescribed nowadays for joint pain. The GP surgery was in the process of stopping NSAIDs for anyone who needed pain relief and advising them to take paracetamol - my GP prescribed 8 paracetamol daily and said it was better to take them routinely as this was better for pain relief than taking them 'as required'.
This, presumably, is the latest protocol.
However, I don't do that as I am not happy about doing so.
I think your MIL's GP should be reviewing patients on a regular basis to ensure that they still need the medication as issuing repeat prescriptions without a regular check-up is not advisable.
I wonder if she should also occasionally (perhaps once a year?) be having a blood test to check liver and kidney function? Also to check for iron levels and other possible issues.
You cannot speak to the GP about this, all you can do is try to persuade her to make an appointment to discuss this with her GP and suggest you accompany her when she goes for the appointment as she may not be able to hear what the doctor says very easily.
Good luck.