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I am an expert with JustAnswer and I am happy to assist you with your question, you can buy out of their share of a property, you have to work out their share of the equity. Typically this involves getting the house valued (the lender will do this, usually for a small fee). Ask your current lender for a redemption certificate to find out how much is left to pay on the mortgage.
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No they are not entitled to more , you would be better off looking for another mortgage and and paying the balance owed.