I have used the cheat to set my relationship at all levels with him. Gone into the bathroom in the evening every single night - it NEVER keys. I did that just to test and see ... my question is : is that code even in? If so - I shouldn't have to be Sherlock Holmes or open the code to find out. I shouldn't even have to have a wiki open to gain traction on the very little provided breadcrumbs.
The number one thing this game could do to be WAY better (again this is my opinion) is to provide WAY better breadcrumbs and reduce the RNG of key interactions.
Getting stepfather to peek at bathroom is not so hard but it's very hard to invite him in.
You need to seduce him in the living room when watching TV together first. And it's a small window to do it, because you need mom and brother not in the living room when stepfather watching TV.
1Tease him till you can seduce him into (anal) sex with you.
2Bathroom peek trigger a lot of times.
Then you can invite him in.
This game has its merit but is very frustrating at the same time. I remember I Sherlock-Holmesed it by open the code to find out like you said.