I would inform the local police that someone has threatened to disrupt your wedding. If any uninvited guests show up, have them arrested for trespassing.
I also think living a stealth life is very stressful. You'll feel a lot more tension if you let people know you are trans. Keeping a secret uses a lot of energy (as you can see by the concerns you post here).
This is something to talk with your fiance about. It's his job to make sure his family treats you with respect, regardless of whether they know your history.
You DESERVE to be treated as the woman you are, even by people who know you're trans. Your fiance needs to understand this, and he needs to make sure his family treats you with the respect you deserve. If he is pressuring you to hide your history from his family, that is worrisome. He should be trying to make life easier for you not pressuring you to keep a secret.