Throwaway for obvious reasons

Okay so I'll give some necessary background information, I'll put an TL;DR at the bottom.

One of my friends is really rich, and the first of us to get married. We'll call her Mia from now on. In our friend group we have 'karen', now Karen has always been a bit defensive and hostile towards Mia. Not enough to have Mia notice this but enough for me to notice.

Mia never made a big deal about being rich, whenever we planned a girls night we would have dinner as places all of us could afford, but she would always cover us if needed. Her gifts for us were always personal, and it took Karen at least a year being in our friend group to discover Mia is rich.

Now upon hearing about Mia's engagement it came to no ones surprise that we would be her bridesmaids. Now I have to say I've known Mia the since high-school so I know that she has been low-key planning her wedding for years. So Mia is going all out, it will be a themed destination wedding. Now before anyone says anything about it being a destination wedding she will pay for anyone's travel expenses, she will also pay for all our custom made dresses.

Now here is the issue were I could the potential a-hole. Karen is one of the bridesmaids and I've heard through some other friends, that aren't invited to the ceremony, they are for the reception that takes place at a later date in our country so more people are able to attend. That Karen is badmouthing Mia to a lot of other friends, saying how she is unreasonable, spoiled and a b-word for wanting her dream wedding apparently. Now Mia has no idea yet, and might throw Karen out of the bridal party if she knows, Mia may be sweet but has a low tolerance for people who are nasty behind her back.

Now of course I don't want Karen to act like this to Mia, I'm not even sure why she is in our close friend group. Although I am not sure if it is my place to tell Mia about this.

So WIBTA if I tell Mia about Karen's behaviour and might get her kicked out of the bridal party.

TL;DR my friend is getting married and generous enough to pay for everything, another bridesmaid is being nasty behind her back and now I want to reveal the truth about her and might get her kicked out of the bridal party. 