First scenario, what was the father's affection? If it reaches 45, he should never ever gain any suspicion during the act. Yes, doing the deeds with him does increase the suspicion.
Second scenario, did you have sex with him in public spaces (i.e Living room/Kitchen)? Also was the brother's suspicion high? If yes to all above then you probably got caught by the brother and then it lead to a bad ending.
Not sure if you're aware but if once suspicion reaches 100, you will get a bad ending.
If you can remember the details leading to the ending then it would be great.
Thanks! Bro contents will be released soon.