I think the Iron Blood already planted a lot of spies within the Namgoong. Which makes them know that the family actually love Ahee, but can't show it publicly because of the Nanny and the other elder's conspiracy plan. Since the head of Namgoong is currently not in a good state after the war, there are probably a lot of internal fight (mostly elders that eyeing the heir position).
If the Iron Blood know that much, that was actually the perfect plan. Namgoong was always described as a clan with deep loyalty and pride. If Ahee was accused of such a thing and was about to snatched away from the family, her siblings won't sit still. The fact that all three of them died, I assume they already prepared this for so long to the point of gathering a bunch of army like they're were about to have a war with a demonic cult or something.
If all the heirs died, and Ahee remains. Namgoong will collapse for sure b3cause she had no cultivation. That was a gold plan actually.
To me, these newer chapters where we see the elders talk with each other, it seems to be implied/hinted that some of them are working for or with other forces to bring Namgoong down completely, and not just seek the position of head of the family / the power it has. If this is the case, framing MC would also make sense - even if it's solely blamed on her, the family would still take a hit, and in the case of the siblings potentially caring for her, their refusal would also hurt Namgoong, giving an excuse to destroy the family. Which was what ended up happening in their previous life.
From what I can see, FL is the only legitimate child of the former head (i.e blood related). Their initial plan was probably to brainwash her then take over Namgung with her as a puppet. When the siblings planned much better than they expected and that failed, they tried to forcibly take her using the poison, my best guess would be that they'd marry her to their eldest son then take over Namgung that way. Her being "useless" is all the better for them, in fact I would guess her being "useless" was caused by them to make her easier to handle. I bet she will be OP as hell in this life bc of superior bloodline or something.
Why did the family's enemies never stop targeting her even after she allegedly became "worthless"? She allegedly had no worth anymore, and even the family ignored her, yet the enemies apparently still thought her useful as a victim/hostage, since they took so many pains to frame her, much later on, in order to try to take her from her family. Suspicious. Why the heck would the enemy bother with a worthless reject? What the heck was everyone hidding from her?