Take her up on it.

"Okay, but not for too long, I do still need to get to know my team and all," Jaune chuckled.

"Sounds good to me~!" Ruby chirped, though already making slightly different plans. She was thinking about how best to bring Jaune into their goals. As they walked towards the library, Yang making sure to exaggerate her steps in a way that swayed her hips and gave her boobs a nice bounce, drawing both Ruby and Jaune's gazes. Ruby walked up to Yang and whispered in her ear, "He's a switch... any ideas for how we can play this?"

Yang turned and whispered back to Ruby, "Well you guys are clearly compatible, if the magic aura vision I gave you works on him... hmmm. But that still doesn't exactly tell you his desires beyond that other than he likes to switch between dominant and submissive roles, possibly an equal role in some cases..."

"So, I don't know his kinks or what would cross a line with him," Ruby mused, thinking on a solution. Yang was right, all the aura vision did was give her a sense of who was compatible with her harem, and would like to be part of it, but it only gave general terms, not exact ones.

"What are you two whispering about?" Jaune asked curiously.

"Oh, nothing, just girlfriend stuff~!" Yang purred, enjoying that Ruby's earlier wish made it perfectly fine to flaunt their sexual relationship despite being half sisters, and she was milking it for all it was worth.

"Oh, okay... I'm not a third wheel, am I?" he asked, blushing a bit as they reached the library doors.

"Only on a tricycle~!" Yang chirped, getting punched in the arm by Ruby.

"Yang! Don't just spring the idea of a threesome on people!" she protested, blushing heavily.

"Sorry, Rubes," Yang chuckled. Jaune blushed deeply.

"...what am I getting myself into?" he asked ruefully.