"The two other siblings and my parents have a pretty close relationship with my mother and father"
huh? are you saying you are adopted?
if you've been with your fiancé for 5 years then what's the problem? She knows them, you know them. Why on earth would you think anything is ever going to change, absent counseling. Especially if you are familiar with mental health.
What you need is a wingman or two for the wedding weekend to ensure no flare-ups and smooth things over. This should be your best man/brother and you need to talk to him and tell him that the wedding weekend you expect him to keep things running smooth and cool and run interference so that you, your wife, and your guests enjoy themselves.
After the weekend, don't send a kiss-off letter to your parents. Either it won't be well-received or it isn't necessary. Just set your boundaries and adhere to them.
Sometimes in these families the parents stay together for the kids. That doesn't mean you got to stay together with the family when you grow up. Sometimes you gotta divorce your parents. Sounds like that is where you are headed. Better be marrying a heckuva girl though because once you have kids your wife will be focused on the little one and you're gonna feel pretty alone.
Good luck