This line is over, we can still meet him in the game, but nothing serious is expected with him
Am more interested in knowing what conflicts that would help further Kasie and her brother(s) relationship. I can see you using Kasie getting jealous of Mia if Mia shows interest in either Derek or Kenny... Still am wondering how your going to show Kasie's inner torment of her conflicting feelings and urges. Personally, though I may be wrong, but I think your more focused on emotional attachment of Kasie more so than her own desires; it's actually a pretty good plot building point.
Still as the artist, your biggest cross to bear is that possible 3-some rout. I don't envy all the work you would have to do if you're going to get to homebase in making 3 distinct alternatives:
..1: Going Solo-- Derek ends up with Kasie. She gives her heart to Derek and eventually sees sex as much part of a relationship as well as accepting pleasure from it. That found condom or a condom could be a good plot point, or a gallery image were Kassie has a choice to interrupt a passionate make-out scene to get a condom or forgo getting one.
..2: Happy trio-- Kasie becomes frustrated. Her own urges and desires overcome her hesitation and moral dilemma. There you get a full on 3-some with Kasie and Derek and Kenny.
..3: Tag team-- Kasie has individual one-on-one with each brother. Kasie becomes so aroused and having feelings for her brother were each of them can get what they want from her and she accepts it willingly. Would be interesting that this rout, should Kasie becomes pregnant that she doesn't know who the father is.
Like what you have done here. After the story is done, you have enough material for either a reboot or added content for a 2.0 version.