Kind of disappointed that as long as the game has been in development the MC still doesn't have any sex scenes with Kate or even Yui. In fact it seems there's still very few vag sex in the entire game. Most of the LI's don't even go all the way with the MC. The whole dungeon area still seems very undeveloped too. Seems there is a whole lot of missed opportunities for not only more events, but overall story development. Maybe in another 1 or more there might actually be more content.
Yui is hardly in the game as the current version 0.9.96.
So its understandable why.
Yui also is very isolated, its written that meek and running away is her personality.
She has only slowly been opening up to the M.C.
Kate.....
You'll get a lot of people with different opinions on that matter.
She seems to be extremely popular, with that being said, she is also family to most people.
(I'm saying this due to you are given the option to state relationship with her)
This could be the reason why its not written, it could also be much like Lynx,
Kate may not want to go to that just yet.
However we at least get internal dialog with Lynx.
Although she could be in a confused state trying to justify her lust/love for her brother.
I can only imagine what kind of internal struggle it would be if you loved your sibling but also felt it was wrong.
Believe me that there is no shortage of people wishing for more Kate.
The 3some question of Her , M.C. and Erick pop's up every now and then.