Marc, sounds like it could be a win/win for you both. Dealing with family could be really good, or really bad, its a risk of mixing family with business for sure.
You have a few options that I see.
1. you all sign an agreement that says you are buying the house subject to the existing mortgage staying in place, and agree on a price, that way you DONT have to get a loan for the full purchase price, you just come out of pocket for the rehab and holding costs. The thing is, you would have to have this money ($20,000-50,000?) in cash/credit/HELOC whatever. You could either close, transfer title and have the existing mortgage in place, or have the agreement spell out how much each party is owed at closing. You flip it, go to closing, the in laws get their purchase price money, and you get the difference as profit. This might create some odd tax scenarios, so consult a CPA for more info.
2. You purchase the house from them with a hard money lender, who will lend you the purchase price and hopefully the rehab money as well. You might need that same amount as above liquid for a down payment on the loan.
Hope that helps so far!!