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Unfortunately no. As long as your mother was of sound mind and competent to sign a contract, there was no fraud or duress on the part of the agreement or undue influence on the part of your sister. If your mother was incompetent at the time then she could not have signed for the personal loan and it would be held to be invalid.
Four elements must be shown to establish undue influence. First, it must be demonstrated that the victim was susceptible to overreaching. Such conditions as mental, psychological, or physical disability or dependency may be used to show susceptibility. Second, there must be an opportunity for exercising undue influence. Typically, this opportunity arises through a confidential relationship. Courts have found opportunity for undue influence in confidential relationships between husband and wife, fiancé and fiancée, parent and child, trustee and beneficiary, administrator and legatee, guardian and ward, attorney and client, doctor and patient, and pastor and parishioner. Third, there must be evidence that the defendant was inclined to exercise undue influence over the victim. Defendants who aggressively initiate a transaction, insulate a relationship from outside supervision, or discourage a weaker party from seeking independent advice may be attempting to exercise undue influence. Fourth, the record must reveal an unnatural or suspicious transaction. Courts are wary, for example, of testators who make abrupt changes in their last will and testament after being diagnosed with a terminal illness or being declared incompetent, especially if the changes are made at the behest of a beneficiary who stands to benefit from the new or revised testamentary disposition.
Nevertheless, courts will examine the facts closely before finding that a transaction has been tainted by undue influence. Mere suspicion, surmise, or conjecture of overreaching is insufficient. The law permits loved ones and confidants to advise and comfort those in need of their support without fear of litigation. Courts are also aware that the doctrine of undue influence can be used as a sword by the vindictive and avaricious who seek to invalidate a perfectly legal transaction for personal gain. When undue influence is found to have altered a transaction, however, courts will make every effort to return the parties to the same position they would have occupied had the overreaching not occurred.
It will be difficult to overturn a transaction such as this and almost impossible for the bank unless you can prove some sort of implicit fraud. Your focus should be on the sister and what type of behavior caused your mother to complete this transaction.
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