Is a brother sinful if he remains silent about an abomination his sisters witnessed, then informs his father after some time, and consequences resulted from his silence?

"Whoever witnesses an evil should change it with their hand or tongue if they are able, unless they fear a greater harm. Ibn al-Qayyim mentioned that enjoining good and forbidding evil has four degrees: The first and second are permissible (the evil is removed or diminished), the third is a matter of ijtihad (it is replaced by something similar), and the fourth is forbidden (it is replaced by something worse). The forbidding of evil is particularly emphasized for a brother concerning his sisters, as they are his honor. If he fears a greater harm, he should inform his parents. Whoever neglects to forbid evil or inform others without fearing a greater harm is sinful. However, if one fears a greater harm and strives to stop the evil and prevent its recurrence, then they are excused."