I had a similar situation as mlk described. My inlaws heard me and dh having a semi-arguement in our bedroom (door closed!) and my mil burst through the bedroom door and tried to hit me. My dh literally had to pick her up and carry her out of the room. After that she proceeded to walk around our living room beating her chest with her fists SCREAMING 'my son! my son! how dare that nasty woman talk back to my son!'.
If this is how your inlaws are interfering now, it's only going to get worse. Your fiance should have had a talk with his parents after you told him how they called you up and mentioned taking the engagement ring back. They over stepped their boundaries by even calling to lecture you. A fight between you and your fiance is none of their business.
You need to have a serious talk with your fiance about what level of interference from his parents you will tolerate. Make it clear you don't want it and that if it crosses the line again, you will expect him to put a stop to it. Afterall, you can TRY to stop them yourself, but I think that rarely works. If your inlaws respected you enough to listen when you say 'stop', well then, they wouldn't be interfering in the first place. People who are rude enough to interfere and talk badly to you at the drop of a hat, are not going to suddenly take your word as law and back off just because
you said so. They'll just simmer for a little while and bounce back. That's why I'm a firm believer in getting a spouse to handle their OWN parents. If a wife's parents were causing problems, I would tell her the same thing. That is just my opinion.
Doing NOTHING often means A LOT. What I mean by that is, when a son or daughter sits quietly and lets their parents abuse their spouse, it sends a message. And that message is, 'I think your behavior is acceptable. Go ahead. Keep doing it, I won't stop you.' That is why anytime a spouse doesn't step in to stop abuse from the outside world, it is always a red flag.