The best way to do this — Don’t.
1- the house should be only on name of my blood relative, not spouse
If my father, or father in law came to me with this I would say “No thank you”.* If you don’t care enough about my wife to have her name on the house then really I don’t want to own it.
Two If you can’t afford to just write a check don’t loan the money. Really The borrower is slave to the lender (from Proverbs, don’t have the exact reference handy) I don’t want that kind of relationship with my folks. It will make dinner taste funny.
*Not that they ever would