We highly admire your level of concern in keeping yourself away from any type of sin, or wrong doing. It is the duty of every Muslim to acquire such a level of knowledge that he or she may fulfil their obligatory acts, as well as to abstain from all types of Haram. As reported from Rasool Allah صلي الله عليه و سلم.
“طلب العلم فريضة علي كل مسلم” رواه إبن ماجة
Translation: Seeking knowledge Is obligatory upon every Muslim.
The Scenario that you have mentioned above will NOT necessitate Hurmate Musahra. Merely having wicked thoughts creep up in your mind of either your father, father in law, or any other family member will not be enough to constitute Hurmate musahra.1
Waswasa
As Muslims, our biggest enemy is Shayton. He is constantly working hard in trying to steer us away from the path of righteousness. He uses many different tools in trying to capture us in his plot. One of the tools Shayton uses is Waswasa. He would often place such thoughts in our mind which causes us to have doubts in all aspects of our religion.
As soon as these waswasa start to emanate, we should try our best to not entertain these thoughts, and immediately indulge ourselves in Zikr. The more we entertain these thoughts and keep them in our mind, the deeper we will fall in shayton’s plot.