Everyone instantly assumes that it was the father who acted as the second, but that may not be correct. Think about it, at the execution, there are four members of the family, the shithead father, the fake sister, the MC, and most forgotten of all, the brother. It has been revealed that the brother and MC were on a very good relationship, and this relationship persisted all the way through to her execution. What if, after the execution and the demon uprising/legendary drought occurs, the father is basically shot, drawn, and quartered, aka for the uninformed, non-fatally shot, hanged until near death, and then killed by being cut into pieces. The fake sister runs away before she can be arrested or something, and only the brother is left. At the execution, he did verbally protest the MC's execution, so perhaps when the extent of what the magic tower can do is revealed as the next duke, he decides to hunt for, and desecrate the MC's tomb. He then brings the MC's body to the magic man and logics it out that he would have no power in the past, the father is a shithead/ was killed before the fake sister was completely proven to be fake, grandparents are dead, and the royalty had no influence on this family matter. Therefore the only possible solution was for the MC to have the memories, however since she was dead, and the was some time until it all went to hell, the dad inadvertently also received some recollections, but never the complete picture, leading to his actions thus far.