You need to have an attorney review the case fully. The I-601 process is quite complicated.
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They should quit receiving dubious advice and consult with an immigration attorney. You can facilitate that on their behalf since you are here. To become eligible to file for I-601 waiver, one needs to convince USCIS that denying immigration to him or her would result in extreme hardship to either spouse or parent of the individual seeking the waiver. As you can see, a sister or fiancee is not included in the list of individuals for whom hardship needs to be proven to exist in order for the waiver to be issued.
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He needs to speak to an immigration attorney asap. There are very good attorneys on this forum and offer consultations on Skype as well. Good luck
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