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I am sorry to hear about this situation. The lender often has a requirement that insurance be paid (this may include hazard insurance). If it is not, the lender can purchase their own and add the sum to the mortgage payment. This is standard.
Unless there is an agreement between the owners in writing as to who should pay the insurance, then there is no mandate that each owner pay their equal share of insurance proportional to their ownership. There is no automatic, statutory division of liability for insurance.
I am guessing no such contract is in writing. If not, then the owners really have little option. They must pay the insurance (one of them most of it, or all of it divided between each other, however they want to), or the lender will add their own insurance and increase the mortgage amount.
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