Owner Financing Question

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Ok, a quick run down of the situation. Any feedback is great. 

My cousin rents a house from her husbands uncle and he is offering them to buy the house through owner financing. My first question is what are the pro's and cons to this oppose to going to a traditional lender? My 2nd question is the uncle has a mortgage out on the house still, can he finance something that he doesn't technically own? 

I will obviously advise her to contact a lawyer, but i wanted to see what feedback I could give her from this great website first. Thanks in advance all.