Added: I looked at G. Y.'s profile. She answered two questions about giving m-in-laws keys in 2011 and 2009. One of the past questions, written by someone else, includes a comment about being bullied. G. mentioned her problem with her mother-in-law when answering someone else's question. Perhaps this is a clue for G.'s current question.
G., perhaps you could let us know why you're asking this question now. The 2 earlier posts that seem to be answered by you had many suggestions. Perhaps you'd get help from those answers.
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How were you bullied? She asked. You could say no. That is not even close to being bullied. Why is she asking you? Your son, her husband has keys. Is there a reason she can't use his?
Several trusted people have keys to my house. A neighbor because I locked myself out a couple of times. A friend, because we watch out for each others house when we're away. A brother, because he lived with me for several years. When they were alive, my parents had 1 key. My Dad would sometimes just walk in unannounced. I asked him to knock or call first. He did. No problem, no drama.
A cousin's husband does not give anyone a key. He was locked out. I now have a key with the understanding I'm only to use it when one of them asks me to use it.
I do not understand why this is a problem. If,for whatever reason you don't want her to have a key, just say no in a respectful way. We really need more info if we are to understand and help you.