@omegaxis1 : Not defending him, just speculating:
He's already the 2nd son, meaning unless he make some kind of grand achievement or the 1st brother die, he's not going to inherit the title and thus be left at the mercy of the brother.
Then if he infers from the letter that the MC's true power will only be revealed at the location, and not doing so would hide her true worth, he can use this.
By disposing of the letter, he can wait until she's isolated by the others, then show some kindness to make her reliant on him.
Then go to that location, unlock her OP power and get to use her however he want, be it as powerful pawn, marry her to another family for favor, or ofc incest.
Scummy and hate-worthy? Yes, but it'd also make more sense on why he would do it.