ok deep breaths Sabrina. First of all, she is in no way stealing your thunder. You are both engaged, thats super exciting to be able to share this with your sister!! I would be thrilled if my sister and I were engaged at the same time.
Second of all, your mom committed to making your dresses first so i am sure she will still be doing that and if your sister wants some made to, then she will have to be behind you.
Third of all, I would just say 'lets go this day to try on dresses for my day, and then we can go this day to try on for yours' if you really think its an issue of trying them on the same day. These are easy fixes, you just have to communicate with her.
When you planned your destination wedding I think you guys knew that you wouldnt have a big wedding like you would have had when you were at home, right? It will still be YOUR day and it will still be wonderful!!!! And so what if her wedding is before yours.. it will be her day and has nothing to do with your day. And after she is married, you will STILL have ac ouple more months being engaged on your own adn then it will be your day.
Just relax and take a deep breath. I truly dont think this is a bad thing at all!! Hugs!