Are you asking about the futa or step sister?Well, others have replied, but yeah basically netorase isn't cheating per se, it's more akin to sharing (MC willingly shares his LI with others, while netorare is cheating, the LI decides to cheat on MC).
I like the threesome idea, the step-sister can be cool, the futa friend too (would it be for the GF only or would the MC be able to play with her too ?)
Either way, the idea I currently have is for my version of Tara to be futa (which would explain why she's shy), and the step sister would be younger and inexperienced
From that point, I actually want to create a side story for them to be together, at least sexually (though I think building an actual relationship would be far more interesting)
As for playing around, step sister + MC and Futa + MC are planned, also threesome of them
Not sure about GF with them, but I'm open to ideas