If your MIL isn't going to go to her GP voluntarily, then another approach you can take is to put together a body of evidence that you can give to the GP to help with diagnosis. Get a diary and record on a daily basis all the areas where she is struggling. Make a note of the time of day that they happen, together with details of any underlying explanations eg was she over tired. You can also use your mobile phone to take pictures or video even. You can then pass along your evidence to the GP or, say, a consultant, when gets referred. The diary is also useful for looking back over time and making note of any deterioration and how quickly it is happening.
Hope this helps,
Fiona