I'm resentful that my future in-laws were collecting on an inherited loan to my fiance
Why is this something that you are upset about?
I don't like anyone who try to squeeze money out of me.
I think you should just leave it alone and let them handle their money the way they want to. Its not your money its hers and her familys.
You should never try to collect money from someone who doesn't want to give it to you. It's people like that that make the world a bad place.
It may be fine to tell your fiance about this, however you should personally leave it alone. If your motivation is to make sure your partner is not being taken advantage of then it makes sense.