There are many, many ways of structuring this. For me, it would be ideal if your in-laws sell the house to you and your spouse for 0 down, with them carrying a note. The sales price, interest rate, and length of the loan could be manipulated so that your monthly payment will be about the same as what you pay now, or the same as what your in-laws get from their current tenants, or something similar.
But if the terms are too generous, your spouse’s siblings won’t be too happy about it.
