I have been backstabbed by the people most close to me whom I considered family :( . The people I'm talking about are my brother-in-law's wife, and her sisters and a couple of friends. For the past 2 years we've been together almost every day. We've been doing all sorts of stuff together and we always have so much fun and I really love them so much and they always seemed like they really loved me and my fiancée too, and they always called us to hang out and they've always cared about us. I can't express how close we were!

A few days ago I accidentally found a group on whatsapp where they were all in except for me, my fiancée and her brother of course :S it was dedicated to hating the two of us they were always talking about us making fun of us in every way. Even the stuff that were not even remotely related to them they gave us stupid names and they kept saying how much they hated us and every time we got together or traveled somewhere on vacation they were on that group talking about us and how they didn't even want us there. The group was created less than 3 months ago, so maybe it wasn't always like that, but I was really shocked because I really loved them and I was there for every one of them through a lot of tough times. I thought they appreciated my help but instead they hate me after all what I've done and I've done A LOT.

Till now they keep pretending like they love us and that they care :s Even when I saw one of them today because we had plans before I saw the group and I didn't want to cancel because I don't know what to do yet. She kept asking me what was wrong because I was angry and it was obvious. She kept asking me all day if I was okay and she called me to check on me afterwards :s . If I didn't see it with my own eyes I would have never believed that they don't care about me, because they act like they care too much or at least they care a bit.

So now I don't know what to do. I can't confront them because beside it being soooooo hard they might not believe that I found that group by accident. I can't tell my fiancée because she will make a very big deal of it. I mean I know it's really huge but I don't want this to cause a big fight between my brother-in-law and his wife because I know it will :s .I certainly can't keep treating them in the same way because they are a bunch of lying hypocrites :( . I can't just not speak to them because we're family and I'll have to see them a lot.

You are not like me because I would have asked the crap posters what the hell they mean.

I understand the shock, so I suggest that you write down your options. Some are:
Confront them angrily from a distance and stop seeing them.
Confront them calmly (puzzled, hurt) and wait for an explanation.
Be distant and let them worry about what you know.
Tell your BIL and let him handle it - it's his wife and her sisters and friends.
Talk about it with your fiancé and her brother, and say you want no part of them anymore, regardless of what they choose to do.

That last one is what I would do.
You say that it isn't possible to not be around them, but you are wrong.

I would very calmly tell them "I came across your group chat and feel insulted and betrayed. It's awkward because we are connected by family. At the moment I have not decided how to handle it, but I'm angry and hurt". If it was posted online, you hardly violated any reasonable expectation of privacy if you found it. If you did, you had good reason to be suspicious and snoop.

The good news... you can talk all of these giant A-holes off your wedding list. If other family ask why just say "It's personal but they know the reason".

Or possibly, just keep it to yourself and go forward seeing them when you have to. You will forever have the upper hand on these folks because you know that they cannot be trusted and you will deal with them accordingly, while they go on figuring that you are clueless.
You will probably have opportunities to help them or make life a little easier for them, but you won't and they will be puzzled.
"Revenge is a dish best served cold"
I don't know if this is a Christian approach or not, but I have used it in many instances especially in the workplace. Folks let their mouths (or keyboard in this day and age) run wild sometimes without realizing how others will interpret their words. They are too stupid to realize that, like votes, words can have serious consequences. It doesn't bother me that things don't always go the way they thought it would go, and they have no idea why their plan didn't work out, but I do.
"Keeps your friends close, and your enemies closer"
I think that I am close to joy's "Be distant and let them worry about what you know" option.
And start nurturing some of your other friendships which will help you tremendously, and give you some good excuses for not being around them very frequently. Also, this let's you hold onto your BIL's friendship as I think that would very possibly be lost with a major blowup where everybody has to choose sides.