It only goes up to +50%, and if you can't crush rebellions before they get out of hand... your revolt risk when nobody is rebelling is too high or you don't have proper response forces in theatre.
Now, in a game as character driven as CK2, it is always possible that something goes disastrously and hilariously wrong with nobody to blame but the RNG, but the most likely reason for spiraling out of control in revolts is inadequate preparation or poor realm management, so think of the "100 opinion" as a
first step rather than the goal itself.
It definitely isn't just anybody in your dynasty.
I don't know the precise mechanics as I only became aware of it recently. I first noticed it when I noticed that one vassal, who'd climbed into the danger zone threatening revolt, got a RR reduction from being my son-in-law. I also think I saw one getting RR for being my son's brother-in-law. This is definitely something that bears closer investigation, but my initial investigation suggests that this is only a select group and hence not all that useful or easy to take advantage of.
It would be considerably easier to e.g. adjust crown laws, since the higher the crown authority, the more the noblemen dislike it and would like to revolt to become free nobles. E.g. high crown authority gives a blanket +10% RR to everybody, no questions asked. Now, reducing crown authority is a serious step and anybody running high or absolute crown authority probably does it for a really good reason, but it
is something to consider seriously for anybody wanting far-flung holdings (and not having all the title holders shacked up near high own capital).