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Boyfriend is coowner of a house with his sister, he wants to sell but she doesn't
He doesn't have to follow through on this, but he could tell this sister that he will stop making mortgage payments, which will result in foreclosure and sale at lower price than might be realized by a voluntary sale. Translation: the house will sold, sis. Do you want to maximize your share of the proceeds?  And, as I said in a comment above: I hope that he is keeping careful records of mortgage an utility payments, as he might (should) be entitled to a refund from the proceeds of an eventual sale (possibly adjusted by the fair rent value of the time which he spent living there)