You have to seriously neglect Kat to get her to cheat on you. For instance if you try to woo Sarah, but also take rime to be with Kat, you are save. If you go for Sarah full speed, the chance of Kat cheating on you rises exponently.and i think i saw somewhere that kat cheated on mc,will it happen if we give her priority?
There is also a netorase/swinging path in the game, where you encourage her to be with other guys. This is not cheating, since you encourage her and can veto some stuff.