Wanna be cruel but gentle about it ? Oxymoron I know, but this will work. But it will take some skill in pulling it off but you can do it because I can tell you are hurt and pissed.
While sitting at the Xmas dinner table with everyone around, casually bring up the topic of technology and specifically phones and it's effects on society where nothing is private anymore but yet soooooooooo easy to be secretive. You then take your exact situation and carbon copy it in a story about a fake friend you have and then tell the exact same details of what this friend found on his or her significant other's phone depending on how you tell the story. You should get a lot of reaction from trust to privacy etc... which is good because it is all generic and gets the group conversation going.
But as you begin to state specifically what this friend found, you look around the table getting everyone's reaction and purposely end your look always looking at her after every detail and asking her every time you state a detail, "Honey can you believe he/she would say that about the person's family he/she is about to marry"? I am so glad you love and appreciate my family as I do yours".
Like I said, you have to be coy about this and very casual but like Frazod said early on in this thread your fiance' is a skilled liar thus she is smart enough to get the message and then you can have your talk on the way home in the car. I always instruct couples to go for drives when discussing sensitive serious topics because either party can't run away and both must maintain control because of the safety factor of driving a car therefore conversations about very serious touchy subjects usually go pretty well. I love road trips because a lot sh** gets aired out that doesn't usually get done at home because of all the distractions and excuses.
Bottom line: Whether she gets it or not during the table discussion, she won't let on that she does nor will she ever be honest with you and admit she feels the same way. Her ego is too prideful because she is a person who really thinks "her sh** don't stink" and your family's stinks to high heaven in her mind and she just tolerates them and you. You then on the way home say, "Hey you know my friend I mentioned at the dinner table? That was me.
Enjoy and make it good because this will end your relationship because her type of personality and character doesn't have the maturity to endure and work through it. Don't waste your time because you don't want a woman like this who will be condescending to your family AND to you as well for the rest of your life. My guess is you have a tendency to date women who always have the upper hand, meaning you like being led by the nose a lot emotionally. I bet you have taken a lot of emotional abuse from women in your past relationships who could take or leave you at anytime? And now you are going to actually marry one ???
Sack up and be a BIG DOG for a change and start respecting yourself and have fun doing this. AND DON'T APOLOGIZE FOR LOOKING AT HER PHONE WHATSOEVER !!! Every time she tries to divert by saying why did you look at my phone? You say, "Why didn't you tell me you felt this way about my family?
And don't back off !!
Have fun !!!