It's quite interesting to see the differences in reactions of the two sisters. If you pick Ariel she celebrates and shows little concern for Hayley, she even considers staying with the MC instead of going to console her sister like they promised. Hayley, on the other hand, immediately comforts Ariel and doesn't even consider staying with the MC instead of consoling her sister. The whole scenario does make Ariel look a bit selfish compared to Hayley, i must admit. Still, after seeing Ariel's reaction to not being picked, i just can't go with Hayley. I don't want to be responsible for breaking her heart like that.
There's definitely arguments for choosing either girl. Hayley understands the situation on the ship better and never gets jealous. She also really cares about her sister so she seems the more likely of the two to go back on the agreement and allow Ariel to be with the MC too. I assume Ariel will also relent (eventually) and agree to share with her sister but i'm having a hard time imagining that right now.