If 2 people own a house together equally, can 1 of them put their share up as collateral for another person to purchase a house without the co-owner's consent?

Question Details: If both signatures are required what would happen if the unknowing owner's signature was forged? I ask because my father put our house up as collateral so a cousin who is notorious for not paying his bills could purchase a house. Both of my parents are equal owners of our house, yet my father did this behind my mother's back, without her knowledge or her signature? Is that legal? Is he allowed to do that? If not, what happens to the contract and, more importantly, our house if it turns out that my mother's signature was forged?

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