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Thank you all for your replies and suggestions. I had thought of the police, but my husband is not keen on the idea. As regards neighbours, she always said that she didn't know her neighbours. They are at work all day, and in any case she is a very disagreeable woman who wouldn't speak to anyone if they tried to be friendly towards her.
I have pleaded with my husband to get down there and make some enquiries in person, but he wont. He point-blank refuses to 'phone his sister, or go to her house, which I would have thought would be the first move. I think I care more about finding my mother-in-law than he does, but she has never liked me and I don't think she would be very pleased to see me, even if I was able to locate her. I think it should be my husband who actually goes in to see her, although I would accompany him on the trip. All that is, of course, assuming we can find her whereabouts.
You are probably right, JackDaniels, about my sister-in-law preventing her mother making contact with my husband. She is a nasty piece of work and I wouldn't put it past her. However, another possibility is that mother-in-law has specifically told her daughter not to tell anyone where she is. If, for example, she has had to go into a care home, she is the sort of person who would consider that shameful and degrading, and wouldn't want anyone to know she was in "one of those places".
Dotty, to answer your question, she will be 100 next February.
I will keep working on my husband and try to get him to agree to a trip down there. I don't think we are going to get any information by 'phone.
BJ