Hello! I'm here for the weekly challenge! As the answer to your question...
Yes, I believe their relationship will evolve. Although Max's sister will get jealous of them getting close, so she will probably do something drastic to get rid of them and immediately regret it later.
The two probably won't date for a long time, but when they do it will be awkward at first. When they finally grow on each other, that's when the sister pulls her stunt!
I wasn't really sure if you were still doing these weekly challenges, but I figured, there is no harm in trying!
If I win, or if you just wanna do it, could you check out my story, "Water"? It is about mermaids;)