@Chaoskunk I don't think she read his mind. The confessing on the island part felt like it was talked over PA or loudly, so slut was chipping in herself.
As for the elder sister, maybe she's already got the "I'm already part of the family, don't need to fight with these bishes for little bro D when I've had it and will continue to have it for as long as I live" mindset. Could be wrong though, I haven't reread this series since I started.