Hehe thanks, their mom will appear later, the path with the boss is over
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Bringing back into the story the mother?
That is that before or after Kasie is in a "real relationship?" If before, you have a timing issue and a roadblock, the mother actually. Getting alone time with any love interest with mom around is going to be difficult to further Kasie's development. If it's after Kasie is an active sexual partner of Derek (and maybe Kenny), you have an open road if the mother becomes a love interest herself. It a matter how that is developed and adds a logical progression.
But first, there is Kasie. Where is she in her emotional state--when will she come to terms with her sexuality with Derek (and/or Kenny)? Ok at this point Kasie is passively involved with Derek. Yes she gets felt up, is more adventurous in appearing nude around Derek (and/or Kenny), has passionately kissed, accidently gotten off once, is involved with some masturbation so on and so forth. As players we know Kasie is more "corrupted" if that is the term to use and Kasie will go just so far at this stage. It's just we do not know Kasie's feelings because Kasie doesn't know her feelings.
However, when Kasie knows her feelings, then there comes the point of acceptance of them. Last, the acting on them becomes the last barrier. Now as a developer you are open to consequences, regrets, fears, joys, happiness, etc. you wish to explore in your project.
Pregnancy issues if you want to explore that aspect. It's unlikely Kasie is on the pill or on any other birth control since up to the boy's visit, none was needed, and Kasie's character isn't the party type girl. An easy work around is the "safe" day and "not safe" day method. Which gives you more content options for plots.
Pair bonding verse polyandry issue arises. If the path is just Kasie and Derek, getting around Kenny to engage in the relationship would be an interesting plot to deal with. The Derek and Kenny path offers three possible plot options for Kasie's character. First, Kasie goes full on up for it; second, reluctant Kasie only wants Derek but is drug into it with Kenny too. And A Kasie meekly but enjoys the situation.
Still going back to the mother coming back into the story. Just a simple word of caution. You have done a great job fleshing out the characters and giving them all personalities, but the flash backs of earlier times make clear the mother is in charge and Kasie is submissive to the mother