I'm going to have to disagree there.
The bad ending is simply way too easy to get if you try to be nice to both of them. Hell, even with the hints in this thread, it took me a couple of tries to eventually get it, and I only got it by a single point with Cassie...
Which also brings to mind, the dominance points from Cassie. Some choices makes very little sense as to why Cassie is getting a dom point (for example
It's also incredibly easy to become a pussy with Cassie, by just trying to please her for some points here and there.
I mean, unless you know some or all of the alternative choices that brings in a bad point first, and more good points later on, it's incredibly difficult not to get the bad ending. Especially since Cassie was teasing the MC right away with fucking the sister... It felt obvious at that time, that she'd be interested in a threesome (you even get points with her by admitting it, right off the bat.)
Hell, being a jerk and enslaving the sister, is much easier... and that makes very little sense to me, it should be a lot more difficult in my opinion ^_^.