You're clearly jealous of Lexi, and possibly Gary...how in anyway is her attraction to your brother "relevant"? Because it makes her a "bad girlfriend"? Believe it or not, women in relationships can still be attracted to other people! Weird, right?! What a slut!
I could go line by line and deconstruct your awful post in order to illustrate what a little immature, butthurt twat you're being, but I don't feel like it.
Instead I'll say this:
If you had a dick(brain? both?) you could've had a conversation with Gary about whatever concerns/issues you have with his relationship and/or extended the olive branch to Lexi. How about explain why it's important for you and he to still maintain a friendship that is separate from theirs? Sometimes an upfront convo solves problems before they exist.
Or, even better, grow the fuck up and realize that in the next 5-10 years all of your "best bros" will likely be married with kids and won't have the time of day for you. This is classic shit, man. Don't be the jealous friend. Instead, go get laid, maybe get your own gal because I guarantee you that you don't currently have one given the nature of this post.
Lastly, you had no business grabbing your buddy's phone and sending that bitchmade, passive aggressive text. If you felt it needed to be said, you should've looked her in the face and told her what you thought. You clearly owe her an apology, no debate there, and the longer you put it off, the more of a dipshit you are.