Question, what are the hidden interactions that this game have? Kinda curious especially after triggering the sister waking up to have sex
Wet dreams - You have 120 lust and going to sleep without doing anything to her. Focus of wet dream is random.
Toilet Masturbation - She have 50+ lust and at least 50 sexual interest goes to the toilet after drinking, chance for this. 100% or near 100% if you used aphrodisiac. With high sexual interest, you can walk in on her for various scenes. (Eat out, piss drinking, blowjob/fingering)
Masturbation tool - She goes to sleep with 100+ lust and points of frustration (a hidden stat), the higher her sexual interest the more she'll do. Will only insert if you've already had sex at least once.
Bathing alone masturbation - She have 50+ lust and around 50 sexual interest you go watch her bath alone, chance she'll finger herself. At 100+, chance she'll do a second round after climaxing. If 100+ and high sexual interest, during her second round if you make cat noise she'll invite you in to watch her up close (may not require it to be second round but that's how I got it to work).
Lake fun - If you've already had sex with her once and her sexual interest is relatively high, you can ask to have sex when you go swimming during the weekend.
Titty peek - After the guild quest to come up with new food recipe, you'll get the option to visit the guild for dinner. If her sexual interest is high enough, she won't refuse you asking to look at her boobs.
This is all I can remember atm, didn't get to hot springs yet and by the time I got to the guild camp part I wasn't in a sexual relationship with her yet so don't know if there's extra stuff during that.