Pre-forclosure help

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I have a stepsister that I just found out is getting ready to loose her house to forclosure in May. I am not sure as to exact numbers yet but am thinking of birddogging this property to investors but have no dealings with what a good pre-forclosure is. I would personally feel that most people in forclosure have little to no equity in the home otherwise they could most likley refinance?(Am I right?) If this is true than how does purchasing a pre-forclosure property profit the investor, when most people in forclosure owe more than the house is worth.