If this were me, my knee-jerk reaction would be, hell no! I would not be happy about it AT ALL! Example...we had uninvited guests at our wedding (my best friend's inlaws, also one of the GMs parents, my best friend and one of the gm's are married)**sorry if that's confusing...I was so upset that they were coming because I felt like it was going to turn into a family reunion for them, not the mention that they were bringing their daughter down as a gift to her and to celebrate her college graduation. I was so mad because I thought they were going to steal my thunder and take away from my day.
In the end, everything turned out just fine (besides the uninvited mother wearing and all white dress at my wedding ceremony lol). I've learned that you can only control what you can control. Express how you feel, keep it classy, and move on. If they want to go against your wishes and get engaged before your wedding day, is there REALLY anything you can do about? I'm afraid there probably is not. Chances are others will be aware of how this situation may make you feel and caution the newly engaged couple to keep their celebrations to a dull roar because, after-all, they are there for your wedding.
In my situation, I expressed my concerns to my best friend and just asked her to keep them in mind while we were in Mexico. On several occassions she reminded her MIL that we were there to celebrate my wedding and they would have "family time" after we (myself and DH) left the resort for our honeymoon.
The best part was I didn't have to make an a$$ out of myself by expressing these feelings all over the place and making sure everyone knew just how annoyed I was. I let the situation speak for itself and everything turned out fine.
Word to the wise...only worry about what you can control and don't worry about what you cannot.
PS- I'm a leo too
Good luck!