This is one of the hottest sister type of games I've had the pleasure of enjoying.
The pros:
1. The sister is crazy hot and has some "pretty realistic" reactions
2. The build-up and progression where you slowly make her accept stuff from you is really well done.
3. The guild stuff was a decent change of pace.
4. The sex/pervert scenes are really well done and look hot.
5. It's a top tier porn game imho.
6. The battles aren't horrible.
7. It's more of the Sister! Seikatsu monochrome (or w.e it was called) which is a pretty great game.
The cons:
1. The grind - you will have to slowly dismantle your sisters walls, which will take a while, and not knowing what's a good thing to try out and what isn't might make you escape and reload saves to progress as soon as possible.
2. The guild stuff is pretty barebones, I haven't had a chance to flirt with any of the girls there, so I guess they're pretty much the "plot" in this game, but even then it' s kinda weak.
3. Unless you save-scum you will be punished by tickling your sister too early or maybe whipping your dick out when she isn't quite interested, or maybe the RNG is against you and your sister catches you going to the to panty treasury. It's unnecessarily punishing and some activities waste precious resources for trying. Would've been nice to get a "are you sure" prompt whenever you're doing something stupid.
4. The general layout, functions, the look of the sister, etc, everything is a bit too similar to the first game, which kinda feels weak for a sequel - it's a great foundation, it has a lot of great stuff and many good nuts to give, but by normal game sequel standards it's kinda bad (but as a porn game it's still delicious).
5. Some skills might not be as good as you might think- At least I think that there are times when you want your sister to be awake (not sure if it's optimal, but her being awake and more ok with stuff kinda works together with the progression system imho, so a skill that makes you better at keeping her asleep doesn't feel like it really has a point (other than the fetish, I guess).
I guess I was quite critical with the cons, but as a porn game, it's just more of the first game, which is a win in my book. Just a bit too close to the first game in terms of character design and general game layout so it feels more like an addition/extension to the first game instead of its own proper standalone porn game (slightly fantasy version of first game).