This doesn't make sense. The basic premise of the story is how everyone is on Eric's side because he's a successful rich man, and everyone thinks nothing of you because you're a loser who stay at home after being kicked out of school.
Manipulated his work life with hacking skills? What hacking skills? Max doesn't even finish school. I think you're running too far away with your imagination for this part. I do wish we get to convert everyone one by one to be against him. It seems only Kira was the one on your side while Ann/Alice/Lisa are all on his side. It would've been satisfying to get them turn against him.
A good way to use Eric IMO is to set a planned event on certain days where you have to be able to counter to prevent his increase influence. The "buy dress/bikini before Day 10" is a good start, but then there's really nothing afterwards. Would've been great if there are more goals to complete (e.g.: bi-weekly lingerie shopping for Alice to use for her blog, monthly clubbing dress for Alice, helping Lisa for exam (harder than homework), more photoshoots with Kira, etc). Then after completing one person's path (let's say starting from Lisa then Alice then Ann), that person will turn to be on your side. Once everyone is against Eric, then the whole family get to kick him out.
Yes, that was me running away with my imagination. I guess Max could have become a hacker with 10 easy online lessons. It doesn't really matter, because that's not the point. That's just me pointing out there are lots of ways it could have been done better than simply vanishing him mysteriously.
Of course we know why the family is on his side at the start, but by mid-game that has changed, and it could have kept changing. As I said above, in my game he didn't have any sex with the sisters, and the aunt is a porn star, so she has sex with other people daily, so that doesn't mean much, but there should be a way to turn Mom away while making Eric our little bitch, rather than being his. We needed more ways to deal with him, using the existing storyline, as I stated, such as the fake ID, the blackmail photo, etc.
I agree, that the dress buying was a good start, and something I had expected to be a mechanic throughout the game. It turns out though that the game is a bit of a random mess of mechanics and the stats don't mean much.