Real Estate doesn't work that way. The OWNER is the person to whom title is vested, (with that person's spouse having a possible marital property interest). It makes no difference who made the payments.
This was a moronic way to handle a RE ownership. You can't possibly have bothered consulting with an attorney or RE professional about what your interests would be when you hatched this plan.
The house is NOT yours, not even your MILs. You have no legal interest. You have no rights. You have nothing but a check register showing paymrents on someone elses property.