Cool idea, to be able to play both sides of the equation! I think it would be beneficial though if the Mom started out actually favoring Chuck: being proud of the successful older brother, she devotes more time to the younger brother out of worry. That gives Chad the motivation to win her over - "I'll show her who's the better son" - and makes the NTR on Chuck's side hit harder - "We were always so close! Why does she look at
him like that now? Why does she seem to consider me a nuisance? What changed?!". It also sets up some antagonism between the brothers, so Chad has a reason to rub his successes in Chuck's face. This dynamic could actually be applied to the sisters too; Chuck is the one the little sis looks up to and the one the older sis doesn't push around (instead, she's cute with him). That makes Chad's conquest more compelling and understandable since he's a guy with a lot to prove. It also makes the NTR on Chuck's side bite harder, since his relationships were so good but now... what happened, Chuck? You've already done this partially by having Chuck being the one with a girlfriend and the positive relationship with the hardline teacher; I say go all the way and apply it to the family too! Everyone likes an underdog story (on the Chad side) and the Champions of Chuck should appreciate the longer, harder fall from grace too.
That aside, there's not much to say about the Chad route just yet. The basic framework is there. As for Chuck, a lot of what's presented in the demo is him being clueless - will that be the norm, or will he catch on and eventually be actively antagonized by Chad? I feel like it would be more interesting on both sides of the equation for Chad to really grind home Chuck's humiliation - it's gratifying for the netori on Chad's side, and the NTR really kicks in when the deep suspicions are finally confirmed for the worst and the helpless all-consuming jealousy portion of the story begins. Chad has to sneak around to start, but once he has leverage - say, "I'll get Mom to kick you out. You know she'd do that for me", "If you expose my relationship with Sis, what would that do to your reputation at school? It tarnishes the whole family, after all... and you are part of the family. Will your students ever respect you again?", or "Go ahead, tell the neighbor what his wife and I are up to. It will tear her apart, and I'll be the only thing she can depend on. You'd be doing me a favor." - with that leverage Chad can start sending Chuck videos, force Chuck to cover for him and a lover, or even tell the lovers so that they become scornful of Chuck and even get off knowing he's watching but cannot do anything about it. I've seen that some NTR fans prefer the MC being a clueless idiot all the way through, but idk... I think the game's strength is playing the two MC's off each other, which won't happen as much if their primary interaction is Chad sending his brother away at every opportunity. Also, what would Chuck's gameplay look like then? Walk around here, go there, come back only to just barely miss Chad's actions? The whole game? Eh.
Ultimately though, the more I think about it, the more I am impressed with the concept. Both sides feed into each other - the glorious triumph of the conqueror directly amplifies the abject humiliation of the loser, which is exactly what fans of either side are looking for. I see from earlier comments that this game is a sideproject... I hope you enjoy making it enough that it one day becomes a priority, because I'm really interested to see how this experiment turns out!
Obligatory prequel meme:
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