My [25M] sister [15F] is dating my ex's [26F] brother [14M] and it's causing massive problems with my fiancee [26F]

I use to date Alicia 6 years ago when I was studying up in Queensland at University. We broke up because I wanted to move back to Melbourne and she didn't. I met my current partner shortly after I got back.
What are the odds that 6 years later Alicia's family would move to town? Not only that but for her brother to go to the exact same high school as my sister and for the two of them to start dating. They've never met each other. He's never met me and my ex's brother never met me. The two of them don't actually know we use to date because our parents told us to keep out mouths shut and not to ruin their fun.
As I said, my parents and Alicia's parents have told us to keep our mouths shut because it's both of theirs first real relationship. It's causing massive problems with my fiancee because she's not happy my ex is back in the picture. She's mad that I had to talk to her for a while when picking my sister up. She's not happy that my sister is dating my ex's brother. What do I do about my Fiancee not being happy about this?

---

tl;dr: My sister has started dating my ex's brother and it's caused lots of fights between myself and my fiance over this. How to resolve it?


Source.

They evidently have trailer parks in Australia