I feel your frustration 🥵
Here are a couple of links that might help you.
First, this is what I call the military marching orders type. It lists the specific steps you can take to get the most gals at the end.
https://www.yekbot.com/sisterly-lust-walkthrough-achievement-guide-harem-ending/
If you find, as I do, that it also takes the fun out of it, here's a link to a walkthrough. I do like these because you can refer to them when you feel stuck or uncertain but you don't have to be a slave to it.
https://www.scribd.com/document/439650484/Sisterly-Lust-walkthrough
That one will show you the walkthough but it won't let you download the PDF without a paid subscription. What I did was simply copy and paste it into Word and save it as a PDF.
The goal here with each gal is to build up trust and corruption. Trust, so that they like you, and corruption so that they'll be open to having sex with you 😆
If you don't have Word, you can just cut and paste the text into Notepad and save it in that form.