Dear Gillian,
You have my utmost sympathy - this is a truly appalling situation for you.
But I'm so glad you can see it is related to some sort of illness - it could be dementia but it really needs a proper medical diagnosis.
You have had good advice here. I think it is essential that your husband visits the doctor and asks him to help, as Sylvia wisely suggested. Perhaps your hubby could ask the doctor to arrange a "check up" on some basis, so your DadIL would bring your MumIL into the surgery? That would allow the doctor a chance to examine her himself.
Perhaps you could ask hubby to ask his Dad to keep a look out at home for some of the things you have supposedly taken?? if any of these things
a spare key from her house ,
a goldwatch ,a bible ,a poem book and a pen 2 horse ornaments were to turn up again (which they will!!) then your FIL might begin to see your MIL has a problem . . . . . ????
Although we don't think of a dementia as a mental illness per se, there is a concept that your story has reminded me of:, family members of some people with mental illness may start to exhibit increasingly bizarre behaviour themselves as, unwittingly, they attempt to make the person's behaviour normal by behaving abnormally themselves.
I think Jennifer has a good point here. Many people, confronted with some sort of mental illness, just cannot accept that this is what it is - especially in a loved one. In the older generation, this is increasingly likely to be so, maybe because many of them feel mental illness (or disability) somehow makes the person a "lesser being".
Or they feel that it is brought on, not by illness, but by "something" the person has done.
As we know, this is absolutely NOT true, but it is hard to change the beliefs of a life time, especially those acquired in early life, probably from one's own parents.
When I am horrified that my own Mum (85) holds such beliefs, I remember that she learnt them as a child from HER parents who grew up in the first years of the twentieth century. Very little was then known about mental illness (or disability) so some very strange and wrong "beliefs" were held.
I can only say that you have my deepest sympathy, and I hope that, in time, this whole horrible mess can be sorted out. In the meantime, these are for you!
{{{HUGS}}}