Bug, Kate's points won't go up beyond 50, when I looked at a save editor, "affectionSISTER" variable increases. Assuming this is indeed the variable for Kate's affection.
And a twist of mystery for the part 2 huh... alright, got me interested now. The only thing I can really complain about is sex scenes are so shallow and short. Good animations but it needs to be hotter, longer, put dialogue while they're going at it. Give different animations and so on. This one improvement is crucial.
Lastly, Sandra on her room at night still says "Do nothing"?