My fiancee has an outstanding loan with her ex-husband. When we get married, will I be financially/legally responsible for that loan?
The only way you would be “responsible” for it would be indirectly. Let’s say she brings this debt into the marriage and for her to be able to pay the debt it requires you to supplement her income to cover expenses. But that’s more of a esoteric discussion rather than reality. Almost no couple brings equal income into the marital partnership anyway.
The only way for you to get on the hook for this old debt would be if you cosigned or helped her to take out a loan to pay it off. Don’t do that. If you do, you have just fallen prey to sexually transmitted debt.