Why does suspicion increase every time I use the dad fuck option? Lust increases +1 but suspicion increases +1 too, which ends up leading to a bad ending.
All lewd actions to these two characters always have suspicion gain. That's how it works. The way to mitigate this is to have max affection with them (this way you'll gain less suspicion).
Once the suspicion is too high, you can give them beer to decrease the suspicion. Also, do not do the hate path if you're not prepared to get game over quickly.
If you don't want to get these suspicion endings. Do the family union quest as soon as possible. Once you start this quest, all suspicion gain will stop.