He just asks her, but takes a bit to get there.

"Speaking of 'falling for it'..." Byleth begins. "I must admit I may not have been entirely honest about why I called you here."

"Oh, is this not about helping me with my language?" Petra replies with a tilt of her head.

"No, we'll do that too. It's just that's not the only thing we'll be doing." Byleth realized trying to dance around the topic was only making things more confusing for Petra. "Sothis, I wish Petra could see you."

Petra looked like she was about to respond, right before Sothis appeared behind Byleth, lounging in the air. "What is - so you can commune with spirits, Professor!" She said, suddenly grinning.

Dorothea simply face palmed. "How is THAT your response, Petra!?" She said in disbelief. Petra looked at her with slight frustration. "I did ask if he could communicate with spirits outside of the large fortress the other day. I am assuming that I 'fell for' the Professor saying no."

Byleth pondered that. "That actually is a valid use of 'fell for' I suppose." He acknowledged. "But not what I wanted to talk about with you. First, how much do you know about genies?"

Turns out only a passing story that had come from a Dagdan trader that had circulated from there, so everyone in the room pitched in to help explain the situation with Sothis. This ended up taking longer than expected whenever Petra raised her hand in confusion, as often about phrases as what Sothis could do. After a half hour, she finally had a decent enough understanding of Sothis for Byleth to finally get to the point.

"The thing is, thanks to Sothis, I have developed an interest in forming a harem. But I also want to keep it secret for now because I don't want to spread any chaos. Dorothea and Manuela are already members, but I was wondering if you wanted to join. If not, we'll simply make you forget about the sex genie stuff and go through a language lesson, then go back to being teacher and student." Byleth laid out, hoping Petra understood enough at this point to make an informed decision...