I can't say much about what type of game I'd like to play, but I can mention a few things from Sleeping Sister that I feel could be improved in the upcoming game.
First is the player character's versatility. I mainly mean that in Sleeping Sister the player didn't have that wide of a choice board and that those few choices were straightforward and to the point. Of course, Sleeping Sister is a small game with a simple premise, so this is a given, but more variety in what the player can do would be a great addition.
The second is vocabulary. I noticed quite a few points in Sleeping Sister where the text seemed identical to other parts of the game. This was most prevalent where the player character orgasms, and the line "Ecstasy surges through your body" is the main thing here. I don't mean to suggest that you need to do anything over the top to amend the vocabulary or phrase usage (for example the synonym system from BDCC), but I do suggest that you look over the text a few times to see if anything feels repetitive.
Last but not least is the feel of the text itself. Sleeping Sister had a great feel to it, and it was put together well, but in my personal opinion, the text was the most lacking part. There wasn't anything inherently wrong with the text or the feel of it, but the flow. Every choice felt less like a transition and more like a block of text being stacked onto the previous block. I can't really describe it well, but to me at least it feels a little... monotone? Every time you wrote in either the player character or the sister showing emotion, you didn't really make them express it, just stated that they were feeling the emotion. I can't think of an example from Sleeping Sister off the top of my head, but I'll make one up. Instead of "You stop when your sister moves a bit, scared that she may have woken up," I would personally write "When your sister flinches at your touch, you freeze up. You watch her with wide eyes, seeing her eyes twitch back and forth under her eyelids." I know it's a small gripe of mine, but I do like descriptive text.
That's all I can really think of in terms of suggestions, but no matter how the new game turns out I'll be here to love it to death just like I love Sleeping Sister. Keep up the good work!