In my first game, I got caught a few times by the brother in the living room for example, so I guess that could explain the problem.
I played a second time and made sure Dad's affection was always 45/45, I confirm the suspicion was still going up, even when I washed the dishes and everything. Maybe I got caught once or twice by the brother, but dad's suspicion got very high at around 90 I think, just before I got the good ending, it was close.
Yes I understand suspicion got me the bad ending, I'm just surprised it went up so fast without warning. I thought there would be a message every time it goes up, like affection, but I can understand if you want this to be a secret thing.