Just the fact Sieglinnde is taking up NetorareNinja's mantle of the previous “Tifa's Dark Desires” makes this at least a 3/5 for me, and it being on Ren'Py streamlines the coom process so much more that it becomes a 4/5 (again, for me). NetorareNinja's game had been on RPGM and though Cloud's job was easy, it had been tedious to do repeatedly. In my opinion, it didn't really add anything to/or maintain the feeling of "What's happening while I'm here" as it became a chore, but Sieglinnde keeps that energy.
Looks wise the large majority is good, models/environment/posing/facial expressions etc. are fairly clean and its issues are minor. In my opinion, the only ones are the outlines/lighting on profiles of characters in nonrendered scenes and Tifa's breasts being visibly low-poly/jagged in her main profile (mostly when clothed, slightly when not) in the inner & top parts.
Story wise, not a lot of resistance from Tifa, but I think we all know why we're here in the first place and it serves its purpose to lead us to the corruption and NTR. I think it might be spiced up if she steps back once she reaches a certain point, kind of like a "What am I doing?" moment.
I can see this being a 4/5 for those who like this kind of NTR/Corruption, but not some aspects of it (maybe it's progressing too fast or the MC is too blind). And 3/5 for those who aren't into the lack of said resistance the heroine has and the way this story moves.
Overall, this is a 5/5 for me and I can't wait for more.