If you're staying with your mother and paying her rent, you're legally a tenant. And what the law says is that a landlord is not responsible for the destruction of a tenant's belongings, unless your mother had somehow done something that caused the fire. (Then she'd be liable for burning your belongings, not as a landlord). Rent usually covers the mortgage, taxes, and insurance, but that doesn't give the landlord any obligation to insure the contents of the property - only the exterior. It unfortunately is your responsibility to have renter's insurance to cover those items, unless you specifically agreed that she would add your belongings to her insurance policy.
Now, if she included the value of your items when submitting a claim to the insurance company, that's fraud. It's actually a crime. The insurance company might be interested to know if that's what happened. But in order to have any sort of claim against your mother, you'd need evidence that she agreed to insure your items. You'd also need to be able to establish what part of the check she got was your belongings instead of hers (or the house itself).
It would be conversion to include the value of your items in the claim to the insurance company and then keep the money. Or, if she told you not to get your own policy, then she'd be liable under a theory of promissory estoppel. In that type of scenario, you'd be able to sue for a portion of the money from the insurance policy.
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