Because having a threesome with sisters counts, since they are both essentially having sex, just with someone else in the mix (don't ask me, thats how patreon seems to work...)
Aldo, the code enables Lucia and Kali content.
It's not incest if they don't have sex with EACH OTHER, even if it's a 3-some.
Of the several sister pairs in the game, MC can only have sex with 2- Kali/Lucy and Felicity/Emma. K/L is a 3-some, but we don't see them doing ANYTHING with each other, and F/E just is a voyageuristic scene where Emma WATCHES Felicity give fellatio. (Or was it the other way round? Clones, you know...) Of the others in the game, sex with Autumn is developing, but MC's relationship with her sister Hazel is poor. We know that Kim & Ally have at least occasional sex with each other, but MC is currently on the outs with them. Lin and Jin share a room, but like K & L, we never see them doing anything. Maria & Hana also share a room now, and we know Maria is bisexual and Hana seems as if she would be open to sex with her, but they are not actually biological sisters.
The incest love hotel is open and there are several couples outside, but no one's going in.
BTW, is there any Kate action AFTER I've answered all of her riddles? I've done it twice now, but nothing happens, other than she wants some time alone. I've already had the pizza restaurant event, but nothing further has happened.