I agree lmao. Unless confidence blinded the him... that itself would be his downfall. Even with a strong family to back him up, a military family is not something to be ignored. To give a royal lineage to a Duke with potential to ruin the throne you're sitting is self infliction at best.
I hate the king but it seems like he's using his sister who he dgaf, and most people find is a burden, but says she's 'precious' to give him many chances to say that the ML's family committed 'treason' for various things so that he can have the excuse to kill them all and wipe them out. ie. You lied to me about her so you commit treason, you let her die in your house it must be treason, etc.
Seems kinda dumb to marry off your sister to the family you KNOW dislikes you and has control over the largest/strongest military in the country. That's like asking for them to usurp you.
They were already trying to plot a rebellion using the king's cousin, but now they have his sister under their control, so it would be super easy to just rebel with her as the potential "rightful ruler." It's even better if she's actually a "half-wit" so they can just manipulate everything behind the scenes and control the country while she's just a figurehead with no real power.
Or, if they are worried about her erratic behavior making them look bad in front of people, they can just make her have a child with the ML, and then use the child as an excuse to rebel and put the kid on the throne (and control the throne as regents while the kid grows up). It seems like it was known that the FML was normal before she was traumatized by seeing her family murdered, so it's not like they have to really worry about the kid also having behavioral issues due to genetics or anything.