The loan you would be looking for is a Home Equity Loan and would be a viable option as long as the lender considers the loan an acceptable risk. That is typically based on your credit worthiness, the value of the home and the amount of the loan. Each lender will have specific guidelines and you can generally get good information from the bigger lender's websites. If your credit is good, that shouldn't be a problem.
Legally, one solution would be to have your in-laws create a trust, make you and your spouse beneficiaries and then give you an interest free loan from the trust. In that case, the loan wouldn't be tied to the house. I think this article has some information that may be helpful.
A word of caution:
This looks great on paper and it may all work out fine, but with such a sizable asset and amount of money, there are many things to consider. Even if everyone gets along great and the future looks bright and cheery, unforeseen situations could arise that change the nature of the agreement or the disposition of one of the parties to the agreement. It's not just about the individuals. There are many external factors that could change things.
It would be best to have a written contract with all of the conditions and expectations so everything is completely clear up front. No chance of any misunderstanding down the road when one person remembers something a little differently than another. There should be extensive thought given to some of the 'what if' types of things as well. No one wants to dwell on it, but nasty things like disability, death and divorce happen in the real world and can greatly complicate handshake agreements.
One thing to be very clear about in the contract is ownership. Do you and your spouse own it and your in-laws have a lien against it like a more typical mortgage? Do your in-laws have partial ownership that gradually transfers to you and your spouse as payments are made? Or is the loan an independent personal loan that is completely unsecured? Will your in-laws have expectations about how the house is maintained and managed or are they content to leave it all to you? If you are unable to pay them back, does ownership revert to your in-laws? Do you have an attorney that can make your preferred ownership arrangement official? Will the legal fees be less than what the loan will cost you?
Even with a very carefully crafted contract, things can go wrong. I am speaking from personal experience with a loan among family members. In our case, there were a number of unexpected turns for the worse over several years. We modified the terms of the agreement multiple times in an attempt to salvage the deal, but in the end, we just scrapped the whole thing and I didn't get much of my money back. I had a written agreement, but the well was dry and it was clear that any effort to recover additional funds would avail nothing but to make a bad situation worse. I am not bitter, love my family members, and have continued to help where I can, but if I had it to do all over again, I would recommend a loan from a traditional lender.