Diana has all the sway and influence needed by MC, her daughter clearly has a huge crush on you and exceptionally naive - why would you not pick the mom and not also expect the daughter to eventually follow suit? She is won over from moment you choose her at party but the mom is clearly no push over and was set as priority target via narrative from the offset so yeah it make far more sense picking her would offer best result in my opinion.
Bit like the princesses - i keep choosing the oldest sister. She is not my fave, the middle one is but i already have her complete attention after the rescue and the youngest is utterly obsessed with me as a character so of course i keep choosing to win over the one who NEEDS one over when it comes to the narrative lol but maybe thats just me. For the most part the game has been kind enough to offer them all on a plate one by one with only the generals daughter really having me think i would have to let her go (i mean endgame for me is supreme ruler - im even seducing the queen herself lol), i cant be having lesser figures dictate my narrative path and ideally there wont be too much having to pick one over other despite increased tensions where even your maid is getting openly jealous and possessive.