Am I petty for this? (Long rant)

So my uncle is my age, like maybe a year older (just a weird coincidence), and his wife and I also had babies about a year apart. The thing about them is, she's so young. She's two years younger than I am, but her maturity is that of a 16year old. She posts on Facebook about all of her marriage issues and complains about him all the time, and always threatens to leave him. They actually just had another kid because they were trying to "fix things". 
Well my fiancé and I have decided to go ahead and set a date for our wedding, and sent out invites and all the fun stuff. And not even a week later, he reproposes to her, and now they're planning an actual wedding (they had a courthouse wedding 2 years ago). Well she's mad because I didn't ask her to be in my wedding (we're not friends and I think my uncle could do better), so now I feel like she's doing this just to be vindictive and annoying. Like, you're already married, unhappily at that. Spending thousands of dollars trying to fix it isn't going to work. I'm so frustrated, and I feel like I have no right to be. Should I even be mad? I feel like she always tries to one up me. Like I became pregnant, she became pregnant. I became pregnant again, she did too (I lost mine), and now this wedding issue. They haven't set a date yet, but I swear if it's close to mine I'm going to lose my shit.